[BRLY-2021-033] SMM memory corruption vulnerability in SMM driver on multiple HP devices.

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-2021-033
• HP PSIRT assigned CVE identifier: CVE-2022-23925
• 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

Potential impact

An attacker can exploit this vulnerability to elevate privileges from ring 0 to ring -2, execute arbitrary code in the System Management Mode - an environment 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 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 e0c277cb-e633-4a8c-b0c6-49e8d685aa30 and located at offset 0x246C in the driver.The pseudocode for this handler is shown below:

EFI_STATUS __fastcall SmiHandler_246C(
        EFI_HANDLE DispatchHandle,
        const void *Context,
        CommBuffer *CommBuffer,
        UINTN *CommBufferSize)
{
  // [COLLAPSED LOCAL DECLARATIONS. PRESS KEYPAD CTRL-"+" TO EXPAND]

  if ( CommBuffer && CommBufferSize && (-(*CommBufferSize != 64) & EFI_INVALID_PARAMETER) == 0 )
  {
    Header = 'UBES';
    Error = -(CommBuffer->Header != 'UBES') & EFI_INVALID_PARAMETER;
    if ( CommBuffer->Header == 'UBES' )
    {
      if ( !CommBuffer->EnableChecks
        || !sub_40E8('UBES', Context)
        || CommBuffer->Case == 2
        || sub_324C()
        || sub_3088(CommBuffer->field_30, CommBuffer->field_28, 0i64) )
      {
        Error = 0i64;
      }
      else
      {
        Error = EFI_ACCESS_DENIED;
      }
    }
    if ( Error )
      goto _Exit;
    Case = CommBuffer->Case;
    if ( Case > 5 )
    {
      Case1 = Case - 6;
      if ( Case1 )
      {
        Case2 = Case1 - 1;
        if ( Case2 )
        {
          Case3 = Case2 - 1;
          if ( Case3 )
          {
            if ( Case3 != 1 )
            {
_Exit1:
              Error = EFI_INVALID_PARAMETER;
              goto _Exit;
            }
            // Case3 = CommBuffer->Case - 8 = 1
            // CommBuffer->Case = 9
            Res = ToCopyMemS(CommBuffer->Offset, CommBuffer->Dst, CommBuffer->DstSize);
          }
          ...
        }
        ...
      }
      ...
    }
    ...
  }
  return 0;
}

Let's assume that the CommBuffer has following structure:

00000000 CommBufferStruc struc ; (sizeof=0x40, mappedto_235)
00000000 Header          dd ?
00000004 field_4         dd ?
00000008 Case            dd ?
0000000C                 db ?
0000000D                 db ?
0000000E                 db ?
0000000F                 db ?
00000010 Offset          dq ? ; Pointer
00000018 Dst             dq ? ; Pointer
00000020 DstSize         dd ? ; Pointer
00000024 EnableChecks    db ?
00000025 field_25        db ?
00000026                 db ?
00000027                 db ?
00000028 field_28        dq ?
00000030 field_30        dw ?
00000032                 db ?
00000033                 db ?
00000034                 db ?
00000035                 db ?
00000036                 db ?
00000037                 db ?
00000038 field_38        dq ?
00000040 CommBufferStruc ends

CommBuffer->Header should be equal to 0x55424553 (UBES).If we set CommBuffer->EnableChecks to 0x00 and CommBuffer->Case to 0x09 than we will trigger the following code:

Res = ToCopyMemS(CommBuffer->Offset, CommBuffer->Dst, CommBuffer->DstSize);

Consider the ToCopyMemS function (offset: 0x1774):

__int64 __fastcall ToCopyMemS(__int64 Offset, void *DstBuffer, unsigned int DstBufferSize)
{
  // [COLLAPSED LOCAL DECLARATIONS. PRESS KEYPAD CTRL-"+" TO EXPAND]

  Status = 0i64;
  if ( !DstBuffer || !DstBufferSize )
    return EFI_INVALID_PARAMETER;
  if ( gProprietaryprotocol1Located )           // Should be zero
    return (*(gProprietaryprotocol_1 + 2))(gArgs[0] + Offset);
  Func = *(gProprietaryprotocol_2 + 2);
  if ( !Func )                                  // Should be zero
  {
    CopyMemS(DstBuffer, DstBufferSize, (Offset + gArgs[0]), DstBufferSize);
    return Status;
  }
  return Func(gArgs[0] + Offset);
}

• the variable gProprietaryprotocol1Located equals 0 because the gProprietaryprotocol_1 protocol with GUID 2452b851-87b5-4225-abf6-c3819cf11256 cannot be located (it is not installed elsewhere)
• below is code from function with offset 0x27C8 that tries to locate the protocol with GUID 2452b851-87b5-4225-abf6-c3819cf11256 (ProprietaryProtocol1Guid):

    Proprietaryprotocol = 0;
    Status = gSmst->SmmLocateProtocol(&ProprietaryProtocol1Guid, 0, &gProprietaryprotocol_1);
    if ( Status )
      Status = gSmst->SmmLocateProtocol(&ProprietaryProtocol2Guid, 0, &gProprietaryprotocol_2);
    else
      gProprietaryprotocol1Located = 1;

• Func equals 0 because gProprietaryprotocol_2 interface (installed in driver 230D with GUID D9E13697-277E-45F0-8F09-841370367FEB) has following structure: <offset Func1, offset Func2, 0>

This means that the CopyMemS function will be executed and data from (Offset + gArgs[0]) will be copied to DstBuffer = CommBuffer->Dst.

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:
2. CommBuffer->Header = 0x55424553
3. CommBuffer->EnableChecks = 0
4. CommBuffer->Case = 9
5. CommBuffer->Dst = {address from SMRAM}
6. 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.

Acknowledgements

BINARLY efiXplorer team