In x86, I get the function address using GetProcAddress()
and write a simple XOR EAX,EAX; RET 4;
in it. Simple and effective. How do I do the same in x64?
bool DisableSetUnhandledExceptionFilter()
{
const BYTE PatchBytes[5] = { 0x33, 0xC0, 0xC2, 0x04, 0x00 }; // XOR EAX,EAX; RET 4;
// Obtain the address of SetUnhandledExceptionFilter
HMODULE hLib = GetModuleHandle( _T("kernel32.dll") );
if( hLib == NULL )
return false;
BYTE* pTarget = (BYTE*)GetProcAddress( hLib, "SetUnhandledExceptionFilter" );
if( pTarget == 0 )
return false;
// Patch SetUnhandledExceptionFilter
if( !WriteMemory( pTarget, PatchBytes, sizeof(PatchBytes) ) )
return false;
// Ensures out of cache
FlushInstructionCache(GetCurrentProcess(), pTarget, sizeof(PatchBytes));
// Success
return true;
}
static bool WriteMemory( BYTE* pTarget, const BYTE* pSource, DWORD Size )
{
// Check parameters
if( pTarget == 0 )
return false;
if( pSource == 0 )
return false;
if( Size == 0 )
return false;
if( IsBadReadPtr( pSource, Size ) )
return false;
// Modify protection attributes of the target memory page
DWORD OldProtect = 0;
if( !VirtualProtect( pTarget, Size, PAGE_EXECUTE_READWRITE, &OldProtect ) )
return false;
// Write memory
memcpy( pTarget, pSource, Size );
// Restore memory protection attributes of the target memory page
DWORD Temp = 0;
if( !VirtualProtect( pTarget, Size, OldProtect, &Temp ) )
return false;
// Success
return true;
}
This example is adapted from code found here: http://www.debuginfo.com/articles/debugfilters.html#overwrite .
In x64 the return value is in RAX, which is the 64bit version of EAX. But because the upper 32 bits are cleared when a 32 bit sub-register is written, "xor eax, eax" is equivalent to "xor rax, rax" and doesn't need to be changed.
However, because the calling convention is different on x64, the same return instruction won't work there: In x86 winapi functions use the stdcall convention, where the callee pops the arguments from the stack (hence the "retn 4" instruction, which pops that one argument from SetUnhandledExceptionFilter off the stack (you may want to fix that comment in your code)). In x64 the stack is not cleaned by the callee, so a normal "retn" instruction needs to be used:
const BYTE PatchBytes[3] = { 0x33, 0xC0, 0xC3 }; // XOR EAX,EAX; RET;
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With