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An example of using conditional breakpoint (events) |
Conditional statements are one of the essential parts of debuggers, and HyperDbg is not an exception.
These statements are handled in both command syntax, like you can specify a core or pid that triggers the event. Still, you can also use the script engine that gives you more flexibility to compare different arguments and, if necessary, call functions.
In this example, we want to put a conditional breakpoint on nt!ExAllocatePoolWithTag only and only when it's called from nt!CmpAllocatePoolWithTag function.
{% hint style="info" %} This example is only applicable to the debugger mode. Because pausing debugger is only available in this mode, however, you can write anything other than pausing debugger in the script in VMI mode. {% endhint %}
Let's take a look at memory at nt!ExAllocatePoolWithTag :
2: kHyperDbg> u nt!ExAllocatePoolWithTag
fffff801`639b1030 48 89 5C 24 08 mov qword ptr ss:[rsp+0x08], rbx
fffff801`639b1035 48 89 6C 24 10 mov qword ptr ss:[rsp+0x10], rbp
fffff801`639b103a 48 89 74 24 18 mov qword ptr ss:[rsp+0x18], rsi
fffff801`639b103f 57 push rdi
fffff801`639b1040 41 56 push r14
fffff801`639b1042 41 57 push r15
fffff801`639b1044 48 83 EC 30 sub rsp, 0x30
fffff801`639b1048 65 48 8B 04 25 20 00 00 00 mov rax, qword ptr gs:[0x0000000000000020]
fffff801`639b1051 45 8B F0 mov r14d, r8d
fffff801`639b1054 44 0F B7 3D A4 8F 34 00 movzx r15d, word ptr ds:[0xFFFFF80163CFA000]
fffff801`639b105c 48 8B EA mov rbp, rdx
The assebly code for nt!CmpAllocatePoolWithTag :
2: kHyperDbg> u nt!CmpAllocatePoolWithTag
fffff801`6325c5c0 48 83 EC 28 sub rsp, 0x28
fffff801`6325c5c4 E8 67 4A 75 00 call 0xFFFFF801639B1030
fffff801`6325c5c9 48 83 C4 28 add rsp, 0x28
fffff801`6325c5cd C3 ret
fffff801`6325c5ce CC int3
fffff801`6325c5cf CC int3
fffff801`6325c5d0 CC int3
As you can see, nt!ExAllocatePoolWithTag is called from fffff801`6325c5c4 in nt!CmpAllocatePoolWithTag.
Based on x86 assembly, a call instruction pushes the address of the next instruction to the stack so that the program can be continued later with the next instruction.
As the stack is down-to-up in computer science, if we dereference @rsp register and read 8-bytes from the stack, it's the pointer to the next instruction that the caller expects to be called after the call is finished (returned).
We know that the next instruction after the call instruction is fffff801`6325c5c9 which is equal to nt!CmpAllocatePoolWithTag+0x9.
Now, we put an EPT hook on the target function nt!ExAllocatePoolWithTag and check whether the caller is nt!CmpAllocatePoolWithTag+0x9 or not.
If the caller is what we expected, then we'll halt the debugger and get the control of the debuggee by using pause(); function.
All in all, the following script is the implementation of this logic.
!epthook nt!ExAllocatePoolWithTag script {
if (poi(@rsp) == nt!CmpAllocatePoolWithTag+0x9) {
pause();
}
}
You can see that the debugger will get the target system's control whenever it's called from nt!CmpAllocatePoolWithTag+0x4.
