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-2021-034
• HP PSIRT assigned CVE identifier: CVE-2022-23926
• 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	015D	1e522785a0a50a4888ea20a91188ed1b40aafdde504d6ee2bd78b389269b4269	29A7E278-1768-42F6-8856-2EB0E013BE67

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 the child SW SMI handler registered with GUID a9878bec-41e6-45fe-8148-2f68c0f7f886 and located at offset 0x1690 in the driver. The pseudocode for this handler is shown below:

EFI_STATUS __fastcall SmiHandler_1690(
        EFI_HANDLE DispatchHandle,
        const void *Context,
        CommBufferStruct *CommBuffer,
        UINTN *CommBufferSize)
{
  // [COLLAPSED LOCAL DECLARATIONS. PRESS KEYPAD CTRL-"+" TO EXPAND]

  if ( CommBuffer && CommBufferSize && (-(*CommBufferSize != 32) & EFI_INVALID_PARAMETER) == 0 )
  {
    Status = EFI_UNSUPPORTED;
    if ( !gComparePointersRes )
    {
      if ( gSmst )
        Status = -(CommBuffer->Case != 0) & EFI_UNSUPPORTED;
      goto _Exit;
    }
    Case = CommBuffer->Case;
    if ( Case )
    {
      Case1 = (Case - 1);
      if ( Case1 )
      {
        Case2 = Case1 - 1;
        if ( !Case2 )
        {
          Status = EFI_NOT_FOUND;
          if ( gFlag )
          {
            CommBuffer->Flag = gFlag;
            Status = 0;
          }
          goto _Exit;
        }
        Case3 = (Case2 - 1);
        if ( Case3 )
        {
          Case4 = (Case3 - 1);
          if ( Case4 )
          {
            if ( Case4 == 1 )
            {
              if ( gSrcBuffer && MEMORY[0xFED40F00] == 0x1050 )
              {
                NewSize = gSrcBuffer;
                DstSize = CommBuffer->DstSize;
                SrcBufferSize = *gSrcBuffer;
                if ( DstSize < SrcBufferSize )
                {
                  Status = EFI_BUFFER_TOO_SMALL;
                }
                else
                {
                  Dst = CommBuffer->Dst;
                  if ( Dst )
                  {
                    CopyMemS(Dst, DstSize, gSrcBuffer + 2, SrcBufferSize);
                    NewSize = gSrcBuffer;
                  }
                  Status = 0;
                }
                CommBuffer->DstSize = *NewSize;
              }
              ...
            }
            ...
          }
          ...
        }
        ...
      }
      ...
    }
    ...
  }
  return 0;
}

Let's assume that the CommBuffer has the following structure:

00000000 CommBufferStruct struc ; (sizeof=0x20, mappedto_234)
00000000 Case            dd ?
00000004 Flag            dd ?
00000008 Status          dq ?
00000010 DstSize         dq ?
00000018 Dst             dq ?
00000020 CommBufferStruct ends

If we set CommBuffer->Case to 0x05 than we can trigger the following code (if MEMORY[0xFED40F00] != 0x1050):

Dst = CommBuffer->Dst;
if ( Dst )
{
  CopyMemS(Dst, DstSize, gSrcBuffer + 2, SrcBufferSize);
  NewSize = gSrcBuffer;
}
Status = 0i64;

The nested pointer CommBuffer->Dst is not validated and a potential attacker could use this to overwrite the contents of SMRAM.

To exploit this vulnerability it is enough to:

1. Setup Communication Buffer:

• CommBuffer->Case = 5
• CommBuffer->Dst = {address from SMRAM}

2. Trigger the SW SMI Handler (SW SMI number and pointer to Communication Buffer are specified in UEFI ACPI table) via 0xB2 IO port

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