[BRLY-2021-040] SMM callout vulnerability in SMM driver on multiple HP devices.

Summary

Binarly REsearch Team has discovered a SMM callout vulnerability on multiple HP devices allowing a possible attacker to hijack execution flow of a code running in System Management Mode. Exploiting this issue could lead to escalating privileges to SMM.

Vulnerability Information

• BINARLY internal vulnerability identifier: BRLY-2021-040
• HP PSIRT assigned CVE identifier: CVE-2022-23932
• 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 REsearch 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 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 malicious 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 807ca9f6-216c-4cbc-87f7-3cd555887208 and located at offset 0x17C8 in the driver.

The pseudocode for this handler is shown below:

EFI_STATUS __fastcall SmiHandler_17C8(
        EFI_HANDLE DispatchHandle,
        const void *Context,
        CommBufferStruct *CommBuffer,
        UINTN *CommBufferSize)
{
  // [COLLAPSED LOCAL DECLARATIONS. PRESS KEYPAD CTRL-"+" TO EXPAND]

  if ( CommBuffer )
  {
    if ( CommBufferSize )
    {
      Size = *CommBufferSize;
      if ( (-(*CommBufferSize != 160) & EFI_INVALID_PARAMETER) == 0 )
      {
        Res = 0;
        if ( Size )
          Res = SmmIsBufferOutsideSmmValid(CommBuffer, Size);
        if ( (-(Res == 0) & EFI_INVALID_PARAMETER) == 0 )
        {
          Status = -(CommBuffer->Sig != 'GFCU') & EFI_INVALID_PARAMETER;
          if ( CommBuffer->Sig == 'GFCU' )
          {
            if ( CommBuffer->Case == 16 )
            {
              if ( !gBufferPtr1 )
              {
                BufferPtr1 = GetCopy(120, &CommBuffer->BufferPtr1);
                BufferSize = CommBuffer->BufferSize;
                BufferPtr = CommBuffer->BufferPtr;
                gBufferPtr1 = BufferPtr1;
                sub_2288(BufferPtr, BufferSize);
                // Vulnerability here
                PcdProtocol = BsLocatePcdProtocol();
                if ( (PcdProtocol->Get8)(0x2C4) == 1 )
                  HandlerUnregister();
              }
            }
            ...
          }
          ...
        }
      }
    }
  }
  return 0;
}

In this case CommBuffer has following structure:

00000000 CommBufferStruct struc ; (sizeof=0xA0, mappedto_234)00000000 Sig             dd ?00000004 Case            dd ?00000008 BufferPtr       dq ?00000010 BufferSize      dq ?00000018 BufferPtr1      dd ?...00000098 Status          dq ?

If CommBuffer->Sig == 0x47464355 && CommBuffer->Case == 16 && !CommBuffer->BufferPtr1 following code will trigger:

BufferPtr1 = GetCopy(120, &CommBuffer->BufferPtr1);
BufferSize = CommBuffer->BufferSize;
BufferPtr = CommBuffer->BufferPtr;
gBufferPtr1 = BufferPtr1;
sub_2288(BufferPtr, BufferSize);
// Vulnerability here
PcdProtocol = BsLocatePcdProtocol();
if ( (PcdProtocol->Get8)(0x2C4) == 1 )
  HandlerUnregister();

Below is the pseudocode of the BsLocatePcdProtocol function:

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

  result = gPcdProtocol;
  // gPcdProtocol equals zero on first call 
  if ( !gPcdProtocol )
  {
    // Executed only once
    gBS->LocateProtocol(&PCD_PROTOCOL_GUID, 0i64, &gPcdProtocol);
    return gPcdProtocol;
  }
  return result;
}

As we can see from this pseudocode, the LocateProtocol service from the EFI_BOOT_SERVICES table is used to get the PCD_PROTOCOL interface. After this, a function from PcdProtocol will be called: (PcdProtocol->Get8)(0x2C4).

Usage of EFI_BOOT_SERVICES and EFI_RUNTIME_SERVICES is unsafe inside a code intended to run in the SMM (from SMRAM), especially in SMI handlers, because an attacker capable of executing code in DXE phase could exploit this vulnerability to escalate privileges to SMM (ring -2).

To exploit this vulnerability it is enough to:

1. Set up a Communication Buffer:
   • CommBuffer->Sig = 0x47464355 ("GFCU")
   • CommBuffer->Case = 16
   • CommBuffer->BufferPtr1 = 0
2. Find EFI_BOOT_SERVICES table in system memory.
3. Overwrite LocateProtocol() service pointer in it with the shellcode address.
4. 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 REsearch Team