MASM64 Peb Walking and Export Resolution | Version | N/A |
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| Updated | |
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| Author | Joshua Finley | License | DBE |
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Using the PEB as a position-independent means of finding exports is not new. As early as the late 90’s, malware authors have been accessing and using the PEB for a variety of purposes [1]. Using it to resolve exports from ntdll.dll, kernel32.dll, etc. has also been documented elsewhere repeatedly [2]. Still, most of the existing examples are in 32-bit assembly, and there are fewer examples in 64-bit. But the world has mostly moved on to 64-bit. Am0nsec [3] demonstrates the most readable and comprehensive approach I have yet found for MASM64, but it is designed specifically for populating tables of function addresses and syscall numbers (the ‘Hells Gate’ tecnique). This article simply descibes a general adaptation of part of [3] - almost all of the code is the exact same, but parts have been changed to make a general-purpose function for iterating over a module’s exports to find a target function’s address.
1. GetExport prototype
We can declare the prototype for this function to be:
// Given the djb2 hash of the target function name and its parent module base addres, resolve its virtual address
__fastcall uint64_ptr GetExport(long djb2_hash, long lpModuleBase);
1.1. Finding export directory iterables
Inside the procedure body, first we need to resolve the following address:
IMAGE_EXPORT_DIRECTORY.AddressOfNamesIMAGE_EXPORT_DIRECTORY.AddressOfFunctionsIMAGE_EXPORT_DIRECTORY.AddressOfNameOrdinals
The HellsGate implementation does all of this for us very nicely:
; in proc GetExport
mov r10d, ecx
mov rbx, rdx
mov r8, rdx
cmp [r8].IMAGE_DOS_HEADER.e_magic, 5A4Dh ; DOS Header --> MZ
jne _getexport_failure
mov ebx, [r8].IMAGE_DOS_HEADER.e_lfanew ; RVA of IMAGE_NT_HEADERS64
add r8, rbx ;
cmp [r8].IMAGE_NT_HEADERS64.Signature, 00004550h ; NT Header --> PE00
jne _getexport_failure
mov ebx, IMAGE_NT_HEADERS64.OptionalHeader ; RVA of IMAGE_OPTIONAL_HEADER64
add r8, rbx ;
cmp [r8].IMAGE_OPTIONAL_HEADER64.Magic, 20bh ; Optional header --> 0x20b
jne _getexport_failure
lea r8, [r8].IMAGE_OPTIONAL_HEADER64.DataDirectory ; First entry of the DataDirectory array
mov ebx, [r8].IMAGE_DATA_DIRECTORY.VirtualAddress ; RVA of IMAGE_EXPORT_DIRECTORY
mov r8, r9 ; ImageBase
add r8, rbx ; Module + RVA
mov ebx, [r8].IMAGE_EXPORT_DIRECTORY.AddressOfNames ; Address of the function name
mov r12, r9 ; Function name RVA
add r12, rbx ; ImageBase + RVA
mov ebx, [r8].IMAGE_EXPORT_DIRECTORY.AddressOfFunctions ; Address of function pointers
mov r13, r9 ;
add r13, rbx ;
mov ebx, [r8].IMAGE_EXPORT_DIRECTORY.AddressOfNameOrdinals ; Address of function ordinals
mov r14, r9 ;
add r14, rbx ;
mov ecx, [r8].IMAGE_EXPORT_DIRECTORY.NumberOfNames ; Total number of named functions
1.2. Search loop
This is where things differ slightly. Instead of identifying predetermined syscall functions and their respective syscall values, we loop over the export table and look for our target (passed in ecx). If we find a function name whose DJB2 hash matches the target, all we have to do is fixup its virtual address and return it in rax.
xor rax, rax ; Set counter to 0
_function_address_search:
mov rbx, 4h ; sizeof(DWORD)
imul rbx, rax ; Get address of next entry
cmp eax, ecx ; Check if rax is still in the range of the export ordinals
jg _getexport_failure ; If exceeded, the export could not be found
mov esi, dword ptr [rbx + r12] ; Set esi to the name RVA
add rsi, r9 ; Add it to the imagebase
push rax ; Store the counter
mov edx, r10d ; Load the target hash
call _djb2_compare ; Check the hashes
mov rdx, rax ; Store the result
pop rax ; Restore the counter
inc ax
test dx, dx
jnz _function_address_search ; If the hashes match, resolve the function's address
_get_function_address:
;mov rcx, 2h ; sizeof(WORD)
;imul rcx, rax ; RCX * sizeof(WORD)
;mov ax, [r14 + rax] ; AX = function ordinal
dec rax
imul rax, 4 ; sizeof(DWORD) * ordinal
mov eax, [r13 + rax] ; RVA of function
mov rbx, r9 ; RBX = ImageBase
add rbx, rax ; RBX = address of function
mov rax, rbx ; Set return value to function address
jmp _getexport_epilog
1.3. DJB2 modifications
This implentation requries a slight modification to the djb2 hashing implementation. We need it to take a target hash and a source string and determine if the latter’s hash is equivalent to the former. This can be done as such:
;-----------------------------------------------------------------------------
; djb2_compare(rcx=target_hash, rsi=source_string);
;-----------------------------------------------------------------------------
_djb2_compare:
push r10
mov rbp, rsp
mov r10d, 5381h ; hash = 0x5381
_djb2_loop:
mov r11d, r10d ; Store original hash value for later
shl r10d, 5 ; hash << 5
add r10d, r11d ; (hash << 5) + hash
xor r11d, r11d ; Clean temporary hash value
mov r11b, byte ptr [rsi] ; Get ASCII char
add r10d, r11d ; ((hash << 5) + hash) + char
inc rsi ; Next string char
cmp byte ptr [rsi], 00h ; End of string
jne _djb2_loop
xor r10d, edx ; Check if hashes match
jz _djb2_match
mov rax, 1
jmp _djb2_epilog
_djb2_match:
mov rax, 0 ; Set status to ERROR_SUCCESS
_djb2_epilog:
mov rsp, rbp
pop r10
ret
1.4. Conclusion
The sum of these parts is a general-purpose function for finding functions from a module’s export directory, without string reliance. Of course, credit for nearly all of these implementation details goes to am0nsec.
1.5. References
[1] https://modexp.wordpress.com/2017/01/15/shellcode-resolving-api-addresses/
[2] https://idafchev.github.io/exploit/2017/09/26/writing_windows_shellcode.html
[3] https://github.com/am0nsec/vx/blob/master/Injector.Win64.HellsGate/HELLSGATE.ASM