[BRLY-2021-037]
SMM memory corruption vulnerability in SMM driver on multiple HP devices.
BINARLY efiXplorer team

Summary

BINARLY efiXplorer team has discovered a SMM memory corruption vulnerability on multiple HP devices allowing a possible attacker to write fixed or predictable data to SMRAM. Exploiting this issue could lead to escalating privileges to SMM.

Vulnerability Information

  • BINARLY internal vulnerability identifier: BRLY-2021-037
  • HP PSIRT assigned CVE identifier: CVE-2022-23929
  • CERT/CC assigned case number: VU#683814
  • CVSS v3.1 8.2 High AV:L/AC:L/PR:H/UI:N/S:C/C:H/I:H/A:H

Affected HP firmwares with confirmed impact by Binarly team

Device/Firmware File Name SHA256 (File PE32 section) File GUID
Device / firmware version: 02.05.01 Rev.A FFFF e16440e9e87d0024404999cfac3806a67ee3ab42631cb810a85dd7cda247308b F66AC4DD-F73D-4252-A6B4-8A48E11FE99C

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

Vulnerability description

The vulnerability exists in child the SW SMI handler registered with GUID 79da6e76-2e45-46e9-b53d-03afbef44844 and located at offset 0x1810 in the driver. The pseudocode for this handler is shown below:

EFI_STATUS __fastcall SmiHandler_1810(
        EFI_HANDLE DispatchHandle,
        const void *Context,
        CommBufferStruct *CommBuffer,
        UINTN *CommBufferSize)
{
  // [COLLAPSED LOCAL DECLARATIONS. PRESS KEYPAD CTRL-"+" TO EXPAND]

  if ( CommBuffer && CommBufferSize && *CommBufferSize == 24 && CommBuffer->BufferPtr && CommBuffer->BufferSize )
  {
    Res = 0;
    if ( CommBuffer->BufferSize )
      Res = SmmIsBufferOutsideSmmValid(CommBuffer->BufferPtr, CommBuffer->BufferSize);
    if ( (-(Res == 0) & EFI_SECURITY_VIOLATION) == 0 )
    {
      if ( LOBYTE(CommBuffer->Case) )
      {
        if ( LOBYTE(CommBuffer->Case) != 1 )
        {
          Status = EFI_INVALID_PARAMETER;
_Exit:
          CommBuffer->Status = Status;
          return 0;
        }
      }
      else
      {
        gPointer = &gData;
      }
      // SMM memory corruption here
      Status = sub_189C(CommBuffer->BufferPtr);
      goto _Exit;
    }
  }
  return 0;
}

In this case the CommBuffer has the following structure:

00000000 CommBufferStruct struc ; (sizeof=0x18, mappedto_235)
00000000 BufferPtr       dq ?
00000008 BufferSize      dd ?
0000000C Case            dd ?
00000010 Status          dq ?
00000018 CommBufferStruct ends

As we can see, the nested pointer CommBuffer->BufferPtr with size CommBuffer->BufferSize is validated (checking that it does not overlap with SMRAM). But CommBuffer->BufferSize is only compared to zero, not the expected fixed value. This means that an attacker can set the CommBuffer->BufferSize value to 1 and then the SmmIsBufferOutsideSmmValid function will check that only the first byte does not overlap with SMRAM.

The pseudocode of the function sub_189C shown below:

EFI_STATUS __fastcall sub_189C(char *Buffer)
{
  // [COLLAPSED LOCAL DECLARATIONS. PRESS KEYPAD CTRL-"+" TO EXPAND]

  DstBuffer = Buffer;
  Status0 = EFI_INVALID_PARAMETER;
  Index = 0;
  if ( Buffer )
  {
    // Change the data pointed to by CommBuffer->BufferPtr
    ZeroMem(Buffer, 0x260);
    Status = EFI_SUCCESS;
    do
    {
      if ( Status )
        break;
      Status = EFI_INVALID_PARAMETER;
      if ( DstBuffer )
      {
        Value = Dref(&gData, gPointer);
        if ( Value == &gData )
        {
          Status = EFI_NOT_FOUND;
        }
        else
        {
          SrcBuffer = Value + 16;
          gPointer = Value;
          if ( Value != -16 && DstBuffer != SrcBuffer )
            // Change the data pointed to by CommBuffer->BufferPtr
            CopyMem(DstBuffer, SrcBuffer, 38);
          Status = EFI_SUCCESS;
        }
      }
      DstBuffer += 38;
      ++Index;
    }
    while ( Index < 0x10u );
    Status0 = Status;
    if ( Status == EFI_NOT_FOUND )
      return 0;
  }
  return Status0;
}

As we can see from the pseudocode, the data pointed to by CommBuffer->BufferPtr is overwritten in two places. Suppose that CommBuffer->BufferSize is 1 and CommBuffer->BufferPtr points to (SMRAM_BASE - 1). It mean that an attacker can potentially corrupt some structures at the beginning of the SMRAM.

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.

Disclosure Activity Date
HP PSIRT is notified 2021-07-12
HP PSIRT confirmed reported issue 2021-08-09
HP PSIRT assigned CVE number 2021-08-19
CERT/CC created a case 2021-11-16
HP PSIRT provide patch release 2022-03-08
BINARLY public disclosure date 2022-03-08

Acknowledgements

BINARLY efiXplorer team