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#include "common.h"
#include "EnforceFilter.h"
////////////////////////////////////////////////////////////////////////////////
//
// EnforceFilter.cpp
//
// Author: Oleg Starodumov (www.debuginfo.com)
//
////////////////////////////////////////////////////////////////////////////////
//
// This example demonstrates how to ensure that nobody else can overwrite
// our custom filter for unhandler exceptions. This is achieved by patching
// SetUnhandledExceptionFilter function.
//
//
////////////////////////////////////////////////////////////////////////////////
// Include files
//
#include <windows.h>
#include <tchar.h>
#include <crtdbg.h>
#include <stdio.h>
#include "EnforceFilter.h"
////////////////////////////////////////////////////////////////////////////////
// Global variables
//
// Patch for SetUnhandledExceptionFilter
const BYTE PatchBytes[5] = { 0x33, 0xC0, 0xC2, 0x04, 0x00 };
// Original bytes at the beginning of SetUnhandledExceptionFilter
BYTE OriginalBytes[5] = {0};
////////////////////////////////////////////////////////////////////////////////
// EnforceFilter function
//
bool EnforceFilter( bool bEnforce )
{
DWORD ErrCode = 0;
// Obtain the address of SetUnhandledExceptionFilter
HMODULE hLib = GetModuleHandle( _T("kernel32.dll") );
if( hLib == NULL )
{
ErrCode = GetLastError();
_ASSERTE( !_T("GetModuleHandle(kernel32.dll) failed.") );
return false;
}
BYTE* pTarget = (BYTE*)GetProcAddress( hLib, "SetUnhandledExceptionFilter" );
if( pTarget == 0 )
{
ErrCode = GetLastError();
_ASSERTE( !_T("GetProcAddress(SetUnhandledExceptionFilter) failed.") );
return false;
}
if( IsBadReadPtr( pTarget, sizeof(OriginalBytes) ) )
{
_ASSERTE( !_T("Target is unreadable.") );
return false;
}
if( bEnforce )
{
// Save the original contents of SetUnhandledExceptionFilter
memcpy( OriginalBytes, pTarget, sizeof(OriginalBytes) );
// Patch SetUnhandledExceptionFilter
if( !WriteMemory( pTarget, PatchBytes, sizeof(PatchBytes) ) )
return false;
}
else
{
// Restore the original behavior of SetUnhandledExceptionFilter
if( !WriteMemory( pTarget, OriginalBytes, sizeof(OriginalBytes) ) )
return false;
}
// Success
return true;
}
////////////////////////////////////////////////////////////////////////////////
// WriteMemory function
//
bool WriteMemory( BYTE* pTarget, const BYTE* pSource, DWORD Size )
{
DWORD ErrCode = 0;
// Check parameters
if( pTarget == 0 )
{
_ASSERTE( !_T("Target address is null.") );
return false;
}
if( pSource == 0 )
{
_ASSERTE( !_T("Source address is null.") );
return false;
}
if( Size == 0 )
{
_ASSERTE( !_T("Source size is null.") );
return false;
}
if( IsBadReadPtr( pSource, Size ) )
{
_ASSERTE( !_T("Source is unreadable.") );
return false;
}
// Modify protection attributes of the target memory page
DWORD OldProtect = 0;
if( !VirtualProtect( pTarget, Size, PAGE_EXECUTE_READWRITE, &OldProtect ) )
{
ErrCode = GetLastError();
_ASSERTE( !_T("VirtualProtect() failed.") );
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 ) )
{
ErrCode = GetLastError();
_ASSERTE( !_T("VirtualProtect() failed.") );
return false;
}
// Success
return true;
}
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