SMM callout vulnerability in SMM driver (SMM arbitrary code execution).

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The vulnerable module registers a child SW SMI handler with GUID 2970687c-618c-4de5-b8f9-6c7576dca83d:

gSmst->SmiHandlerRegister)(SmiHandler, &UNKNOWN_PROTOCOL_2970687C_GUID, &Handle);

The handler is located at offset 0x19D0:

EFI_STATUS __fastcall SmiHandler(EFI_HANDLE DispatchHandle, const void *Context, void *CommBuffer, UINTN *CommBufferSize)
{
  if ( CommBuffer && CommBufferSize && !gExitBootServicesFlag2EfiEventLegacyBootFlag )
  {
    if ( !gUnknownProtocol74d936fa )
    {
      if (gEfiBootServices->LocateProtocol)(&UNKNOWN_PROTOCOL_74D936FA_GUID, 0, &gUnknownProtocol74d936fa) < 0 )
        return 0;

As we can see before EFI_EVENT_EXIT_BOOT_SERVICES is triggered, the handler will try to locate protocol 74d936fa-d8bd-4633-b64d-6424bdd23d24 using the EFI_BOOT_SERVICES which is located outside of SMRAM, hence this call could be hijacked by a possible attacker.

Despite the vulnerability cannot be exploited from the operating system, using EFI_BOOT_SERVICES and EFI_RUNTIME_SERVICES is unsafe inside a code intended to run in SMM (from SMRAM) because an attacker capable of executing code in DXE phase could exploit this vulnerability to escalate privileges to SMM (ring -2).

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This bug is subject to a 90 day disclosure deadline. After 90 days elapsed or a patch has been made broadly available (whichever is earlier), the bug report will become visible to the public.

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BINARLY efiXplorer team

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