[BRLY-2021-014] SMM memory corruption vulnerability in SMM driver on Fujitsu device (SMRAM write).

Summary

BINARLY efiXplorer team has discovered a SMM memory corruption vulnerability in Fujitsu devices allowing a possible attacker to write data to SMRAM. Exploiting this issue could lead to escalating privileges to SMM.

Vulnerability Information

• BINARLY internal vulnerability identifier: BRLY-2021-014
• CERT/CC assigned case number: VU#796611
• Fujitsu PSIRT assigned CVE identifier: CVE-2022-28806
• CVSS v3.1 8.2 High AV:L/AC:L/PR:H/UI:N/S:C/C:H/I:H/A:H

Affected Fujitsu 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 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 UEFI firmware (for example, Secure Boot and some types of memory isolation for hypervisors).

Vulnerability description

In the function at offset 0x83EC (image sha256: 5c4e7948a36e140de59cfff4697cb58220e90a8a1a7be5121a1cbf8463cb73fb), the child software System Management Interrupt (SWSMI) handler with GUID 2bcf654b-7164-4aec-8bf8-ff830b756799 is registered:

__int64 sub_800083EC()
{
  // [COLLAPSED LOCAL DECLARATIONS. PRESS KEYPAD CTRL-"+" TO EXPAND]

  Handle = 0;
  Status = gSmst_D780->SmiHandlerRegister(SmiHandler_82C4, &gSmiHandlerGuid, &DispatchHandle);
  if ( Status >= 0 )
    return gEfiBootServices->InstallProtocolInterface(&Handle, &ProprietaryProtocol_D288, EFI_NATIVE_INTERFACE, 0);
  return Status;
}

Find below the decompiled SWSMI handler code:

EFI_STATUS __fastcall SmiHandler_82C4(
        EFI_HANDLE DispatchHandle,
        const void *Context,
        CommBufferStruct *CommBuffer,
        UINTN *CommBufferSize)
{
  // [COLLAPSED LOCAL DECLARATIONS. PRESS KEYPAD CTRL-"+" TO EXPAND]

  Status = EFI_SUCCESS;
  if ( !CommBuffer )
    Status = EFI_DEVICE_ERROR;
  if ( !CommBufferSize )
    Status = EFI_DEVICE_ERROR;
  CommBuffer1 = CommBuffer->byte0;
  if ( CommBuffer1 != 1 )
    return EFI_INVALID_PARAMETER;
  dword19 = CommBuffer->dword19;
  Result = sub_80009CB0(
             CommBuffer->qword9,
             CommBuffer->qword11,
             &dword19,
             CommBuffer->qword1D,
             CommBuffer->dword25,
             &v5,
             &v5 + 2);
  CommBuffer->dword1 = Result;
  CommBuffer->word7 = v5;
  CommBuffer->word5 = WORD2(v5);
  CommBuffer->dword19 = dword19;
  return 0;
}

This shows that if first QWORD from CommBuffer is 0x01 then following content from the CommBuffer can be overwritten:

• 4 bytes at offset 1
• 2 bytes at offset 5
• 2 bytes at affset 7
• 4 bytes at offset 25

There is no pointer validation carried out (to ensure CommBuffer and any other Communication Buffer nested contents are not pointing to SMRAM contents). Thus, a potential attacker can write fixed data to SMRAM to corrupt some data inside this memory (for example, change SMI handler's code or modify Smram Map structures to break input pointer validation for other SMI handlers, hence to completely make this mitigation inefficient). This could lead to gaining arbitrary code execution in SMM.

To fix this vulnerability, it is essential to wrap all the input pointers (including the nested pointers) for 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