How do I patch a Windows API at runtime so that it to returns 0 in x64?
- by Jorge Vasquez
In x86, I get the function address using GetProcAddress() and write a simple XOR EAX,EAX; RET; 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;
// 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 .
