x64 Hooks


Author: David Zimmer
Date: 09.19.12 - 1:22pm



Still playing around with hook engine implementations.

on x64 Its a little messy.

jmp data deref - 14 bytes
ff 25 00 00 00 00        jmp [rip+addr0]  
80 70 8e 77 00 00 00 00  data: 00000000778E7080

jmp eax - 12 bytes
48 b8 80 70 8e 77 00 00 00 00   mov rax, 0x00000000778E7080  
ff e0                           jmp rax
A safe 14 bytes at a function start without any jxx or calls feels like a a lot to ask. I had some failures just in my small hooking needs. 12 bytes is better, but really i want to get it down more.

I had my code spit out the prealignment bytes count i used in the x86 "micro" hooks, seems like 8-10 is average for x64 functions. This is enough for an 8 byte address. to utilize the prealignment bytes we could do a "6 byte" jmp[rip-14] style hook such as:
0000000011000000 8 data bytes in function prealignment paddding...
0000000011000008 FF 25 F2 FF FF FF jmp qword ptr [11000000h]  <-- 6 byte hook in API prolog 
You can actually also pull off a 6 byte push ret hook IF the address you are transferring execution to can be represented by a 32bit number.
68 DDCCBBAA      PUSH AABBCCDD    
C3               RETN    

bool is32BitSafe(ULONG_PTR value){
	ULONG_PTR b = value & 0x00000000FFFFFFFF;
	return value == b ? true : false;
}                  
On x64 push can only hold a 32bit constant, however when it is pushed to teh stack, a 64bit number is written with the high 8 bytes set to 0. TO make this work, you can use the following pragma directive to force your applications base address to be 32 bit addressable so this hook type could work.
#pragma comment( linker, "/BASE:0x8000000")
Thats a pretty easy thing to forget though, and might not always work out for dlls, so I guess I need to keep looking for smaller x64 hooks still. Trying to utilize the prealignment bytes comes next I guess.

Actually generating some statistics on common dlls should come next...

btw you can test these with a stub such as:
 unsigned char data[15] = {
	0x0E, 0x00, 0x00, 0x11, 0x00, 0x00, 0x00, 0x00, 0xFF, 0x25, 0xF2, 0xFF, 0xFF, 0xFF, 0xCC
 };
 char* buf = (char*)VirtualAlloc((void*)0x11000000 , 10, MEM_RESERVE | MEM_COMMIT , 0x40);
 memcpy(buf,&data[0],15);
 int (*sc)();
 sc = (int (*)())&buf[8]; 
 (int)(*sc)();
Experimentation is messy stuff...




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