[BRLY-2021-017] SMM callout vulnerability in combined DXE/SMM on Fujitsu device (SMM arbitrary code execution)

Summary

BINARLY efiXplorer team identified a SMM callout in a Fujitsu device, which allows an attacker to access the System Management Mode and execute arbitrary code.

Vulnerability Information

• BINARLY internal vulnerability identifier: BRLY-2021-017
• CERT/CC assigned case number: VU#796611
• Insyde PSIRT assigned CVE identifier: CVE-2021-41839
• 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 could potentially 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 0x1AC0 (image sha256: f6d7d5cce1a3b2ebe9825c195f61f36aca674e3710abf0e24f1383e6d3c0e126), the child software System Management Interrupt (SWSMI) handler with the GUID EFI_NVM_EXPRESS_PASS_THRU_PROTOCOL_GUID is registered:

if ( SmstIsNotNull() )
{
  DispatchHandle = 0;
  gSmst_62B0->SmiHandlerRegister(SmiHandler_17D4, &EFI_NVM_EXPRESS_PASS_THRU_PROTOCOL_GUID_5AF0, &DispatchHandle);
  Registration = 0;
  result = gSmst_62B0->SmmRegisterProtocolNotify(&ProprietaryProtocol_5B80, Function, &Registration);
  if ( result < 0 )
    return result;
}

Find below the decompiled SWSMI handler code:

__int64 __fastcall SmiHandler_17D4(
        EFI_HANDLE DispatchHandle,
        const void *Context,
        void *CommBuffer,
        UINTN *CommBufferSize)
{
  // [COLLAPSED LOCAL DECLARATIONS. PRESS KEYPAD CTRL-"+" TO EXPAND]

  if ( !CommBuffer || !CommBufferSize )
    return 0;
  if ( *CommBuffer != 1 )
    return EFI_UNSUPPORTED;
  SourceSize = 0;
  if ( (gBS_6278->LocateHandleBuffer(
          ByProtocol,
          &EFI_NVM_EXPRESS_PASS_THRU_PROTOCOL_GUID_5AF0,
          0,
          &NoHandles,
          &SourceSize) & 0x8000000000000000) != 0 )
    return EFI_NOT_FOUND;
  Buffer = SourceSize;
  if ( SourceSize )
  {
    gBS_6278->FreePool(SourceSize);
    Buffer = 0;
    SourceSize = 0;
  }
  BufferSize = 0;
  Status = gSmst_62B0->SmmLocateHandle(
             ByProtocol,
             &EFI_NVM_EXPRESS_PASS_THRU_PROTOCOL_GUID_5AF0,
             0,
             &BufferSize,
             Buffer);
  if ( Status )
  {
    if ( Status == EFI_BUFFER_TOO_SMALL )
    {
      gSmst_62B0->SmmAllocatePool(EfiRuntimeServicesData, BufferSize, &SourceSize);
      if ( !SourceSize )
        return EFI_OUT_OF_RESOURCES;
      Status = gSmst_62B0->SmmLocateHandle(
                 ByProtocol,
                 &EFI_NVM_EXPRESS_PASS_THRU_PROTOCOL_GUID_5AF0,
                 0,
                 &BufferSize,
                 SourceSize);
    }
    else if ( Status != EFI_NOT_FOUND )
    {
      return EFI_NOT_FOUND;
    }
  }
  Res = Status == EFI_NOT_FOUND ? 0 : BufferSize >> 3;
  gSmst_62B0->SmmFreePool(SourceSize);
  if ( Res == NoHandles )
    return 0;
  result = sub_428();
  if ( result >= 0 )
    return 0;
  if ( result == EFI_NOT_STARTED )
    return 0;
  return result;
}

If CommBuffer and CommBufferSize pointers are not-NULL and first QWORD from CommBuffer is equal to 0x01, the following code will be executed:

if ( (gBS_6278->LocateHandleBuffer(
        ByProtocol,
        &EFI_NVM_EXPRESS_PASS_THRU_PROTOCOL_GUID_5AF0,
        0,
        &NoHandles,
        &SourceSize) & 0x8000000000000000) != 0 )
  return EFI_NOT_FOUND;

Using services from EFI_BOOT_SERVICES (in this case LocateHandleBuffer()) inside a SW SMI handler is unsafe and may lead to arbitrary code execution in SMM.

In addition to this code, this SMI handler uses the following services in the EFI_BOOT_SERVICES table:

• SmiHandler() -> gBS->FreePool()
• SmiHandler() -> sub_428() -> gBS->LocateHandleBuffer()
• SmiHandler() -> sub_428() -> gBS->HandleProtocol()

Each of these services can be targeted by a potential attacker.

To exploit this vulnerability is enough to:

• overwrite the LocateHandleBuffer(), FreePool() or HandleProtocol() service address in the EFI_BOOT_SERVICES table with the shellcode address
• trigger the SWSMI handler (SwSmi number is specified in UEFI ACPI table)

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