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69c938f Dec 13, 2014
101 lines (92 sloc) 5.51 KB
;-----------------------------------------------------------------------------;
; Author: Stephen Fewer (stephen_fewer[at]harmonysecurity[dot]com)
; Compatible: Windows 7, 2008, Vista, 2003, XP, 2000, NT4
; Version: 1.0 (24 July 2009)
; Size: 130 bytes
;-----------------------------------------------------------------------------;
[BITS 32]
; Input: The hash of the API to call and all its parameters must be pushed onto stack.
; Output: The return value from the API call will be in EAX.
; Clobbers: EAX, ECX and EDX (ala the normal stdcall calling convention)
; Un-Clobbered: EBX, ESI, EDI, ESP and EBP can be expected to remain un-clobbered.
; Note: This function assumes the direction flag has allready been cleared via a CLD instruction.
; Note: This function is unable to call forwarded exports.
api_call:
pushad ; We preserve all the registers for the caller, bar EAX and ECX.
mov ebp, esp ; Create a new stack frame
xor eax, eax ; Zero EAX (upper 3 bytes will remain zero until function is found)
mov edx, [fs:eax+48] ; Get a pointer to the PEB
mov edx, [edx+12] ; Get PEB->Ldr
mov edx, [edx+20] ; Get the first module from the InMemoryOrder module list
next_mod: ;
mov esi, [edx+40] ; Get pointer to modules name (unicode string)
movzx ecx, word [edx+38] ; Set ECX to the length we want to check
xor edi, edi ; Clear EDI which will store the hash of the module name
loop_modname: ;
lodsb ; Read in the next byte of the name
cmp al, 'a' ; Some versions of Windows use lower case module names
jl not_lowercase ;
sub al, 0x20 ; If so normalise to uppercase
not_lowercase: ;
ror edi, 13 ; Rotate right our hash value
add edi, eax ; Add the next byte of the name
loop loop_modname ; Loop until we have read enough
; We now have the module hash computed
push edx ; Save the current position in the module list for later
push edi ; Save the current module hash for later
; Proceed to iterate the export address table,
mov edx, [edx+16] ; Get this modules base address
mov ecx, [edx+60] ; Get PE header
; use ecx as our EAT pointer here so we can take advantage of jecxz.
mov ecx, [ecx+edx+120] ; Get the EAT from the PE header
jecxz get_next_mod1 ; If no EAT present, process the next module
add ecx, edx ; Add the modules base address
push ecx ; Save the current modules EAT
mov ebx, [ecx+32] ; Get the rva of the function names
add ebx, edx ; Add the modules base address
mov ecx, [ecx+24] ; Get the number of function names
; now ecx returns to its regularly scheduled counter duties
; Computing the module hash + function hash
get_next_func: ;
jecxz get_next_mod ; When we reach the start of the EAT (we search backwards), process the next module
dec ecx ; Decrement the function name counter
mov esi, [ebx+ecx*4] ; Get rva of next module name
add esi, edx ; Add the modules base address
xor edi, edi ; Clear EDI which will store the hash of the function name
; And compare it to the one we want
loop_funcname: ;
lodsb ; Read in the next byte of the ASCII function name
ror edi, 13 ; Rotate right our hash value
add edi, eax ; Add the next byte of the name
cmp al, ah ; Compare AL (the next byte from the name) to AH (null)
jne loop_funcname ; If we have not reached the null terminator, continue
add edi, [ebp-8] ; Add the current module hash to the function hash
cmp edi, [ebp+36] ; Compare the hash to the one we are searching for
jnz get_next_func ; Go compute the next function hash if we have not found it
; If found, fix up stack, call the function and then value else compute the next one...
pop eax ; Restore the current modules EAT
mov ebx, [eax+36] ; Get the ordinal table rva
add ebx, edx ; Add the modules base address
mov cx, [ebx+2*ecx] ; Get the desired functions ordinal
mov ebx, [eax+28] ; Get the function addresses table rva
add ebx, edx ; Add the modules base address
mov eax, [ebx+4*ecx] ; Get the desired functions RVA
add eax, edx ; Add the modules base address to get the functions actual VA
; We now fix up the stack and perform the call to the desired function...
finish:
mov [esp+36], eax ; Overwrite the old EAX value with the desired api address for the upcoming popad
pop ebx ; Clear off the current modules hash
pop ebx ; Clear off the current position in the module list
popad ; Restore all of the callers registers, bar EAX, ECX and EDX which are clobbered
pop ecx ; Pop off the origional return address our caller will have pushed
pop edx ; Pop off the hash value our caller will have pushed
push ecx ; Push back the correct return value
jmp eax ; Jump into the required function
; We now automagically return to the correct caller...
get_next_mod: ;
pop edi ; Pop off the current (now the previous) modules EAT
get_next_mod1: ;
pop edi ; Pop off the current (now the previous) modules hash
pop edx ; Restore our position in the module list
mov edx, [edx] ; Get the next module
jmp short next_mod ; Process this module