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12/15/10

[MASM] Calling functions without importing them

I started off writing this as an add-on to my PE Infection post. However, I think it's turned into a big enough project that it deserves it's own thread lol. The original idea was to have a stub for my PE infecter that could call any API it wanted *without* using fixed addresses. Using fixed Addresses makes the resulting stub version-dependant, which is lame.

Anyways, the way this works is pretty simple. It starts off by finding the base address of kernel32.dll (by using the Process Environment Block), and then it parses kernel32.dll's exports and looks for the two functions LoadLibraryA() and GetProcAddress(). From there, any WinAPI function is within arms reach.

Implementing this was another story. The only part that really needed to be in asm was the part that found kernel32.dll, but I chose to write it all in asm. The biggest thing that screwed me up was converting RVAs to plain old VAs. For a while I didn't realize that it was as trivial as adding the RVA to the actual base address, so I ended up writing a function that scanned every section in the PE file and returned the offset the RVA would have in a file. Once I got rid of this, the rest came quickly. :-)

Code:

CODE: SELECT ALL
; ------------------------------------------------
; call.asm - finds kernel32.dll in memory and
; searches it's export table for LoadLibraryA()
; and GetProcAddress(). From there, calling
; functions that haven't been imported is trivial.
;
; :-)
;
; Assembled with:
; nasm call.asm -fwin32 -o call.obj
; gcc call.obj -o call.exe
;
; AdhesiveBall - August/2010
; ------------------------------------------------

[BITS 32]
global _WinMain@16



section .data
WorkedMessage db "Hello from an unimported function!", 0

LoadLibraryName db "LoadLibraryA", 0
LoadLibraryAddr dd 0xFFFFFFFF

GetProcAddressName db "GetProcAddress", 0
GetProcAddressAddr dd 0xFFFFFFFF

MessageBoxName db "MessageBoxA", 0
MessageBoxAddr dd 0xFFFFFFFF

User32Name db "user32.dll", 0
User32Addr db 0xFFFFFFFF

Kernel32Addr dd 0xFFFFFFFF

section .text

; -----------------------------------------------
; void *FindKernel32Base(void)
; tries to find the base address of kernel32.dll
; returns NULL on error
; -----------------------------------------------
FindKernel32Base:
mov eax, [fs:0x30] ; address of PEB
mov eax, [eax + 0x0C] ; PEB->Ldr
mov eax, [eax + 0x1C] ; Ldr->InMemoryOrderModuleList (first element is ntdll.dll)
mov eax, [eax] ; InMemoryOrderModuleList->Flink (second element is kernel32.dll)
mov eax, [eax + 0x08] ; DllBase
cmp word [eax], 'MZ' ; check for MZ header
je FindKernel32Base_end
xor eax, eax ; zero eax if not found
FindKernel32Base_end:
ret


; ------------------------------------------------------
; void *LoadFunction(HMODULE module, char *FunctionName)
; attempts to find the address of function by
; walking module's Export Address Table
; ------------------------------------------------------
LoadK32Function:
push ebp
mov ebp, esp
sub esp, 12 ; make room for 3 DWORDS

; look at my stack frame, my stack frame's amazing
;------------------------------------
; +-----------------------+
; ebp-0x0C | PE Header |
; +-----------------------+
; ebp-0x08 | Export table addr |
; +-----------------------+
; ebp-0x04 | AddressOfNames |
; +-----------------------+
; .....
; +-----------------------+
; ebp+0x08 | kernel32 base address |
; +-----------------------+
; ebp+0x0C | function name |
; +-----------------------+
;------------------------------------



mov edi, [ebp + 0x08] ; edi = DosHeader

; -------------------------
; verify MZ and PE headers
; -------------------------
cmp word [edi], 'MZ'
jne LoadK32Function_fail
mov edx, edi
add edx, [edi + 0x3C] ; module + DosHeader->e_lfanew (edx = PeHeader)
cmp word [edx], 'PE'
jne LoadK32Function_fail
mov [ebp - 0x0C], edx ; save the PE header

; ------------------------------
; find the real addr of the EAT
; ------------------------------
mov eax, [edx + 0x78] ; OptionalHeader.DataDirectory[0].VirtualAddress
add eax, dword [ebp + 0x08] ; add the offset to the base address
mov [ebp - 0x08], eax ; save it!

; ----------------------------------------
; find the real address of export names
; ----------------------------------------
mov eax, [eax + 0x20] ; eax is still addr of EAT (0x20 = offset to ADdressOfNames)
add eax, dword [ebp + 0x08]
mov [ebp - 0x04], eax


; --------------------------------
; start looking for names!
; --------------------------------
xor ecx, ecx
LoadK32Function_loop_names:
mov edx, [ebp - 0x08] ; EAT
cmp ecx, [edx + 0x18] ; NumberOfNames
jge LoadK32Function_fail

; --------------------------------------
; find the address of the function name
; --------------------------------------
mov ebx, [ebp - 0x04] ; AddressOfNames
mov ebx, [ebx + ecx * 4] ; RVA of string
add ebx, [ebp + 0x08]

; compare 'em!
push dword [ebp + 0x0C] ; FunctionName
push ebx ; name of entry
call strcmp
cmp eax, 1
je LoadK32Function_found_api

inc ecx
jmp LoadK32Function_loop_names

LoadK32Function_found_api:
;------------------------------------------------------
; success! now all that's left is to go from the
; AddressOfNames index to the AddressOfFunctions index
; -----------------------------------------------------

; First thing's first, find the AddressOfNameOrdinals address
mov eax, [ebp - 0x08]
mov eax, [eax + 0x24] ; AddressOfNameOrdinals offset
add eax, [ebp + 0x08]

; Now we gotta look up the ordinal corresponding to our api
xor ebx, ebx
mov bx, [eax + ecx * 2] ; ecx * 2 because it's an array of WORDS

; Next we find the AddressOfFunctions array
mov eax, [ebp - 0x08]
mov eax, [eax + 0x1C] ; AddressOfFunctions offset
add eax, [ebp + 0x08]

; and last we find the address of our api!
mov eax, [eax + ebx * 4]
add eax, [ebp + 0x08]

jmp LoadK32Function_end
LoadK32Function_fail:
xor eax, eax
LoadK32Function_end:
mov esp, ebp
pop ebp
ret 8








; -------------------------------------------------------------
; unsigned int RVAToFileOffset(
; IMAGE_NT_HEADERS *ntHeader, unsigned long virtualAddr);
;
; Converts a relative virtual address to an offset in the file
; -------------------------------------------------------------
;RvaToFileOffset:
; push ebx
; mov eax, [esp + 0x08] ; eax = ntHeader
; mov ebx, [esp + 0x0C] ; ebx = virtualAddr
; push ecx
; push edx
; xor ecx, ecx
; mov cx, [eax + 0x06] ; ecx = number of sections
; lea edx, [eax + 0xF8] ; edx = first section
; RvaToFileOffset_loop_sections:
; push ecx ; used as a temporary variable throughout the loop
;
; ; -------------------------------------------------------------
; ; check if the virtual address resides in the current function
; ; -------------------------------------------------------------
; cmp ebx, [edx + 0x0C] ; if(virtualAddr <>VirtualAddress) continue;
; jl RvaToFileOffset_continue
; mov ecx, [edx + 0x0C]
; add ecx, [edx + 0x08] ; ecx = section->VirtualAddress + section->VirtualSize
; cmp ebx, ecx ; if(virtualAddr >= ecx) continue;
; jge RvaToFileOffset_continue
;
; RvaToFileOffset_DEBUGLABEL:
; ; -------------------
; ; Success! It's here
; ; -------------------
; sub ebx, [edx + 0x0C] ; find the offset from the beginning of the section's virtual addr
; add ebx, [edx + 0x14] ; add it to the PointerToRawData member
; jmp RvaToFileOffset_success
;
;; RvaToFileOffset_continue:
; add edx, 0x28 ; edx += sizeof(IMAGE_SECTION_HEADER)
; pop ecx ; restore for use as a counter
; loop RvaToFileOffset_loop_sections
;
; xor eax, eax ; baww
; jmp RvaToFileOffset_end
;
;RvaToFileOffset_success:
; pop ecx ; ecx will be on the stack 'coz of the loop so we need to fix it
; mov eax, ebx
;RvaToFileOffset_end:
; pop edx
; pop ecx
; pop ebx
; ret 8










; -------------------------------------------------
; int strcmp(char *a, char *b)
; returns 1 if the two strings are equal, 0 if not
; -------------------------------------------------
strcmp:
push ebx
strcmp_loop:
mov eax, [esp + 0x08]
mov al, byte [eax]
mov ebx, [esp + 0x0C]
cmp al, byte [ebx]
jne strcmp_not_equal
cmp al, 0
je strcmp_equal
inc dword [esp + 0x0C]
inc dword [esp + 0x08]
jmp strcmp_loop

strcmp_not_equal:
mov eax, 0
jmp strcmp_end

strcmp_equal:
mov eax, 1

strcmp_end:
pop ebx
ret 8


; -----------------------------------------------
; kinda bloated but meh does it's job
_WinMain@16:
call FindKernel32Base
cmp eax, 0
je _WinMain_failed
mov [Kernel32Addr], eax

; find LoadLibrary()
push LoadLibraryName
push dword [Kernel32Addr]
call LoadK32Function
cmp eax, 0
je _WinMain_failed
mov [LoadLibraryAddr], eax

; find GetProcAddress()
push GetProcAddressName
push dword [Kernel32Addr]
call LoadK32Function
cmp eax, 0
je _WinMain_failed
mov [GetProcAddressAddr], eax

; load user32.dll
push User32Name
call [LoadLibraryAddr]
cmp eax, 0
je _WinMain_failed
mov [User32Addr], eax

; find MessageBox()
push MessageBoxName
push dword [User32Addr]
call [GetProcAddressAddr]
cmp eax, 0
je _WinMain_failed
mov [MessageBoxAddr], eax

push 0
push WorkedMessage
push WorkedMessage
push 0
call [MessageBoxAddr]

_WinMain_worked:
; push 0
; push WorkedMessage
; push WorkedMessage
; push 0
; call _MessageBoxA@16
; jmp _WinMain_end
_WinMain_failed:
; push 0
; push FailedMessage
; push FailedMessage
; push 0
; call _MessageBoxA@16
_WinMain_end:
ret 16


I could still improve a couple things. For one, it's probably a good idea to hash the api names and compare the hashes instead of using strcmp. Also, I know that strcmp is bloated (as is WinMain, but that's not really vital).

I might update this in the future with different methods of finding the base address of kernel32.dll, too. I know there are at least two other documented methods of finding it.

Anyways, tell me what you think, and constructive criticism is welcome as always :).
Why is Stefan standing on my shoulders...

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