[BRLY-2021-047] SMM callout vulnerability in SMM driver on HP device (SMM arbitrary code execution).

Summary

BINARLY efiXplorer team identified SMM callout on HP device, which allows a attacker to access the System Management Mode and execute arbitrary code.

Vulnerability Information

• BINARLY internal vulnerability identifier: BRLY-2021-047
• HP PSIRT assigned CVE identifier: CVE-2022-31641
• CVSS v3.1: 7.5 High AV:L/AC:H/PR:H/UI:N/S:C/C:H/I:H/A:H

Affected Intel firmwares with confirmed impact by Binarly team

Potential impact

An attacker can exploit this vulnerability to elevate privileges from ring 0 to ring -2, execute arbitrary code in System Management Mode - an evironment more privileged than operating system (OS) and completely isolated from it. Running arbitrary code in SMM additionally bypasses SMM-based SPI flash protections against modifications, which can help an attacker to install a firmware backdoor/implant into BIOS. Such a malicious firmware code in BIOS could persist across operating system re-installs. Additionally, this vulnerability potentially could be used by malicious actors to bypass security mechanisms provided by UEFI firmware (for example, Secure Boot and some types of memory isolation for hypervisors).

Vulnerability description

The vulnerability exists in child SW SMI handler registered with GUID 807ca9f6-216c-4cbc-87f7-3cd555887208 and located at offset 0x17DC in the binary.The pseudocode for this handler is shown below:

EFI_STATUS __fastcall SmiHandler(
        EFI_HANDLE DispatchHandle,
        const void *Context,
        void *CommBuffer,
        UINTN *CommBufferSize)
{
  // [COLLAPSED LOCAL DECLARATIONS. PRESS KEYPAD CTRL-"+" TO EXPAND]

  if ( CommBuffer )
  {
    if ( CommBufferSize )
    {
      Length = *CommBufferSize;
      if ( (-(*CommBufferSize != 160) & EFI_INVALID_PARAMETER) == 0 )
      {
        Res = 0;
        if ( Length )
          Res = SmmIsBufferOutsideSmmValid(CommBuffer, Length);
        if ( (-(Res == 0) & EFI_INVALID_PARAMETER) == 0 )
        {
          Status = -(*CommBuffer != 'GFCU') & EFI_INVALID_PARAMETER;
          if ( *CommBuffer == 'GFCU' )
          {
            switch ( *(CommBuffer + 1) )
            {
              case 0x10:
                if ( !gBuffer )
                {
                  Buffer = GetCopy(0x78, CommBuffer + 24);
                  BufferSize = *(CommBuffer + 2);
                  BufferPtr = *(CommBuffer + 1);
                  gBuffer = Buffer;
                  sub_261C(BufferPtr, BufferSize);
                  PcdProtocol = LocatePcdProtocol();
                  if ( (PcdProtocol->Get8)(0x23B) == 1 )
                    FreeAndUnregister();
                }
                break;
              ...
              default:
_InvalidParameter:
                Status = EFI_INVALID_PARAMETER;
                break;
            }
          }
          *(CommBuffer + 19) = Status;
        }
      }
    }
  }
  return 0;
}

Consider the case 0x10 branch:

If gBuffer is not NULL (since gBuffer is initialized only within this branch, it will always be NULL the first time it is run), the protocol interface EFI_PCD_PROTOCOL will be located (via gBS->LocateProtocol call) and the Get8 function from the protocol will be called.

The pseudocode of the LocatePcdProtocol function is shown below:

PCD_PROTOCOL *LocatePcdProtocol()
{
  // [COLLAPSED LOCAL DECLARATIONS. PRESS KEYPAD CTRL-"+" TO EXPAND]

  result = gPcdProtocol;
  if ( !gPcdProtocol )
  {
    gBS->LocateProtocol(&PCD_PROTOCOL_GUID, 0i64, &gPcdProtocol);
    return gPcdProtocol;
  }
  return result;
}

This means that a potential attacker could overwrite the gBS->LocateProtocol pointer in the EFI_BOOT_SERVICES table with the shellcode address and trigger this handler. As a result, it will execute arbitrary code in SMRAM.

If the EFI_PCD_PROTOCOL interface is already located (gPcdProtocol is not NULL), a potential attacker could overwrite not the gBS->LocateProtocol pointer, but the gPcdProtocol->SetBool pointer.

Disclosure timeline

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.

Acknowledgements

BINARLY efiXplorer team