[BRLY-2022-013] SMM memory corruption vulnerability in SMM driver on HP device (SMRAM write).

Summary

BINARLY efiXplorer team has discovered an SMM memory corruption vulnerability in an HP device 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-2022-013
• HP PSIRT assigned CVE identifier: CVE-2022-31646
• 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 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 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 SMM driver registers a child software System Management Interrupt (SW SMI) handler with GUID 1ad9b513-5e5e-4242-802a-98191292a7c2, which contains the actual vulnerability:

gSmst->SmiHandlerRegister(SmiHandler, &gSmiHandlerGuid, &SmiHandlerDispatchHandle);

The SMI handler itself is located at offset 0x1D90 in the driver:

EFI_STATUS __fastcall SmiHandler(EFI_HANDLE DispatchHandle, const void *Context, void *CommBuffer, UINTN *CommBufferSize)
{
  ...
  
  if ( CommBuffer )
  {
    if ( CommBufferSize )
    {
      if ( !(-(__int64)(*CommBufferSize != 0x288) & EFI_INVALID_PARAMETER) )
      {

As we can see the specified size of input Communication Buffer should be equal to 0x288 bytes. Hence, its validated part (validation routine is in PiSmmCommunicationSmm SMM module) is 0x288 bytes.

        SmmAllocateZeroWrapper((char **)&gCommBufferCopy, *CommBufferSize);
        CommBufferCopy = gCommBufferCopy;
        if ( gCommBufferCopy )
        {
          if ( *CommBufferSize_1 && gCommBufferCopy != CommBuffer )
          {
            memcpy(gCommBufferCopy, CommBuffer, *CommBufferSize);
            CommBufferCopy = gCommBufferCopy;
          }

So 0x288 bytes allocated in SMRAM allow to copy entire CommBuffer contents.The code below will process WStrings (Unicode strings) in original CommBuffer, with CommBufferCopy[0x282] = iterations (WStrings) count, 0 to 0xFF:

          counter = 0;                       
          if ( CommBufferCopy[0x282] )           
          {
            offset = 0;
            do
            {

Here a pointer to a WString inside the original CommBuffer (not to its copy) is retrieved, then wstrlen() invoked on the pointed WString. The limit is 1000000 WORDs!The string processed inside the original CommBuffer, which should be 0x288 bytes only! Iterations count is controllable! So, we have multiple OOB reads:

              str_ptr = *(_WORD **)&CommBuffer[offset + 0x18];
              if ( str_ptr )                    
              {                                 
                strlen_inwords = GetWStringLengthInWords(str_ptr, 1000000);
                *(_QWORD *)(offset + v11 + 0x18) = 0;
                CommBufferCopy = gCommBufferCopy;
                strlen_inbytes = 2 * strlen_inwords + 2;
                buffer = (char **)((char *)gCommBufferCopy + offset + 0x18);
                if ( buffer )
                {
                  status = SmmAllocateZeroWrapper(buffer, 2 * strlen_inwords + 2); // pointless allocation of a buffer (not used further)
                  CommBufferCopy = gCommBufferCopy;
                }
                else
                {
                  status = EFI_INVALID_PARAMETER;
                }

Below wstrcopy() is invoked to copy the pointed WString from CommBuffer and the limit is the same = 1000000 WORDs! WString is copied into an SMRAM-copy of CommBuffer! Allocation size was 0x288 bytes! So, we have multiple OOB writes:

                if ( !status )
                {                               
                  WStringCopy(                  
                    *(_QWORD *)&CommBufferCopy[offset + 0x18], // dest
                    2 * strlen_inbytes,                        // size
                    *(_WORD **)&CommBuffer[offset + 0x18]);    // src
                  CommBufferCopy = gCommBufferCopy;
                }
              }
              
              ++counter;
              offset += 0x20;
            }
            while ( counter < CommBufferCopy[0x282] );
          }
        }
      }
    }
  }
  return 0;
}

The implementation of wstrlen():

unsigned __int64 __fastcall GetWStringLengthInWords(_WORD *a1, unsigned __int64 a2)
{
  unsigned __int64 result; // rax

  if ( !a1 || !a2 )
    return 0;
  result = 0;
  if ( *a1 )
  {
    while ( result < a2 - 1 )
    {
      if ( !a1[++result] )
        return result;
    }
    result = a2;
  }
  
  return result;
}

The implementation of wstrcopy():

MACRO_EFI __fastcall WStringCopy(__int64 ptr1, unsigned __int64 size, _WORD *ptr2)
{
  unsigned __int64 strlen_inwords; // rax
  unsigned __int64 size_1; // rdx
  char *ptr1_1; // r10 mov     r10, rcx
  char *ptr2_1; // r11 mov     r11, r8
  __int16 val; // ax
  signed __int64 v9; // r11

  if ( !ptr1 || !ptr2 || size > 1000000 || !size )
    return EFI_INVALID_PARAMETER;
  strlen_inwords = GetWStringLengthInWords(ptr2, size);
  if ( size_1 <= strlen_inwords )
    return EFI_BUFFER_TOO_SMALL;
  if ( ptr2_1 > ptr1_1 )
    goto LABEL_19;
  if ( ptr1_1 < &ptr2_1[2 * strlen_inwords + 2] )
    return EFI_ACCESS_DENIED;
  if ( ptr2_1 >= ptr1_1 )
  {
LABEL_19:
    if ( ptr2_1 < &ptr1_1[2 * size_1] )
      return EFI_ACCESS_DENIED;
  }
  val = *(_WORD *)ptr2_1;
  if ( *(_WORD *)ptr2_1 )
  {
    v9 = ptr2_1 - ptr1_1;
    do
    {
      *(_WORD *)ptr1_1 = val;
      ptr1_1 += 2;
      val = *(_WORD *)&ptr1_1[v9];
    }
    while ( val );
  }
  *(_WORD *)ptr1_1 = 0;
  return 0i64;
}

So a possible attacker could overflow the heap buffer allocated for the copy of Communication Buffer with the strings data (arbitrary data with a controllable end - 0x0000 and iterations count in CommBufferCopy[0x282]). This could lead to corrupting heap structures and data/code stored next to it with a potential gaining arbitrary code execution.

This leads to corrupting memory in SMRAM at a controllable address with a predictable data and size. It could be used for example to change SMI handler's code or modify SMRAM Map structures to break input pointers validation for other SMI handlers, hence to completely make this mitigation inefficient. This could lead to gaining arbitrary code execution in SMM.

To exploit this vulnerability it is enough to specify in a CommBuffer:1. Set CommBuffer to contain these values: *(QWORD *)CommBuffer = SMRAM_BASE - 1 and *size_ptr = 1.2. Call SW SMI (SwSmi number is specified in the UEFI ACPI table) via 0xB2 IO port, prior to it SMI handler GUID 67511ba8-5a25-4bba-ac59-f00feb85dffa and Communication Buffer size should be specified in Communication Buffer.

It should be noted that this SMI handler will be unregistered in a callback-notifier for SMM protocol EDKII_SMM_READY_TO_BOOT_PROTOCOL:

Smst->SmmRegisterProtocolNotify(
           &EDKII_SMM_READY_TO_BOOT_PROTOCOL_GUID,
           EfiSmmReadyToBootProtocolNotifier,
           &Handle);

EFI_STATUS __fastcall EdkIISmmReadyToBootProtocolNotifier(const EFI_GUID *Protocol, void *Interface, EFI_HANDLE Handle)
{
  if ( gSmiHandlerDispatchHandle )
  {
    gSmst->SmiHandlerUnRegister(gSmiHandlerDispatchHandle);
    gSmiHandlerDispatchHandle = 0;
  }
  return 0;
}

Due to this fact this vulnerability cannot be exploited from the operating system. However, there is a time window before the above mentioned event will occur and the handler will be unregistered. A possible attacker capable of executing code in DXE phase could exploit this vulnerability to escalate privileges to SMM (ring -2).

To fix this vulnerability, it is essential to wrap all the input pointers (including the nested pointers) of the SMI handlers with sanity checks to make sure they are not pointing into 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.

Acknowledgements

BINARLY efiXplorer team