[BRLY-2022-042] SMM callout vulnerability in SMM driver on AMD-based Gigabyte devices.

Summary

The BINARLY efiXplorer team has discovered an SMM callout vulnerability on a Gigabyte device allowing a potential attacker to hijack execution flow of code running in the System Management Mode. Exploitation of this issue could lead to escalation of privileges to SMM.

Vulnerability Information

• BINARLY internal vulnerability identifier: BRLY-2022-042
• AMD PSIRT has assigned CVE identifier: CVE-2023-20559
• CVSS v3.1 7.5 High AV:L/AC:H/PR:H/UI:N/S:C/C:H/I:H/A:H

Affected Gigabyte firmware with confirmed impact by the BINARLY efiXplorer team

Potential impact

An attacker can exploit this vulnerability to elevate privileges from ring 0 to ring -2, resulting in the execution of 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. This can help an attacker to install a firmware backdoor/implant into the BIOS. Such 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 the UEFI firmware (for example, Secure Boot and some types of memory isolation for hypervisors).

Vulnerability description

The vulnerability exists in the SW SMI handler registered with SwSmiInputValue 0xBC and located at offset 0x1680 in the AmdCpmGpioInitSmm SMM module.

This handler is registered as follows:

__int64 __fastcall sub_121C(EFI_SMM_BASE2_PROTOCOL *a1, __int64 a2)
{
  __int64 result
  EFI_STATUS v3;
  EFI_HANDLE DispatchHandle[3];
  EFI_SMM_BASE2_PROTOCOL *EfiSmmBase2Protocol;
  __int64 InSmram;
  EFI_SMM_SW_REGISTER_CONTEXT RegisterContext;
  EFI_SMM_SW_DISPATCH2_PROTOCOL *EfiSmmSwDispatch2Protocol;

  InSmram = a2;
  EfiSmmBase2Protocol = a1;
  result = gBS->LocateProtocol(&EFI_SMM_BASE2_PROTOCOL_GUID, 0, &EfiSmmBase2Protocol);
  v3 = result;
  if ( result >= 0 )
  {
    EfiSmmBase2Protocol->InSmm(EfiSmmBase2Protocol, &InSmram);
    if ( InSmram )
    {
      result = gBS->LocateProtocol(&AMD_CPM_TABLE_SMM_PROTOCOL_GUID, 0, &gAmdCpmTableSmmProtocol);
      if ( result < 0 )
        return result;
      (*(gAmdCpmTableSmmProtocol + 72))(60);
      gSmst->SmmLocateProtocol(&EFI_SMM_SW_DISPATCH2_PROTOCOL_GUID, 0, &EfiSmmSwDispatch2Protocol);
      RegisterContext.SwSmiInputValue = 0xBC;
      v3 = EfiSmmSwDispatch2Protocol->Register(
             EfiSmmSwDispatch2Protocol,
             SwSmiHandler,
             &RegisterContext,
             DispatchHandle);
      (*(gAmdCpmTableSmmProtocol + 72))(61);
    }
    return v3;
  }
  return result;
}

Consider the pseudocode of the SwSmiHandler:

EFI_STATUS __fastcall SwSmiHandler(
        EFI_HANDLE DispatchHandle,
        const void *Context,
        void *CommBuffer,
        UINTN *CommBufferSize)
{
  // local variables initialization

  sub_1300();
  v4 = gAmdCpmTableSmmProtocol;
  if ( gAmdCpmTableSmmProtocol )
  {
    v5 = (*(gAmdCpmTableSmmProtocol + 113))(gAmdCpmTableSmmProtocol, '11A$');
    if ( v5 )
    {
      for ( i = (v5 + 16); *i != 255; i += 2 )
        (*(v4 + 92))(v4, *i, i[1]);
    }
  }
  return 0;
}

Where sub_1300 contains the following code snippets:

AmdCpmTableSmmProtocol = gAmdCpmTableSmmProtocol;
v1 = 0;
v2 = (*(gAmdCpmTableSmmProtocol + 113))(gAmdCpmTableSmmProtocol, '71A$');
v3 = (*(AmdCpmTableSmmProtocol + 113))(AmdCpmTableSmmProtocol, '30A$');
v4 = (*(AmdCpmTableSmmProtocol + 96))(AmdCpmTableSmmProtocol) & 2;
if ( v3 )
{
  if ( v4 )
  {
      if ( v2 )
      {
          LOBYTE(v5) = -117;
          v6 = (*(AmdCpmTableSmmProtocol + 126))(AmdCpmTableSmmProtocol, v5);
          LOBYTE(v7) = -118;
          v8 = v6;
          v9 = (*(AmdCpmTableSmmProtocol + 126))(AmdCpmTableSmmProtocol, v7);
          LOBYTE(v10) = -119;
          v11 = (v8 << 8) + v9;
          v12 = (*(AmdCpmTableSmmProtocol + 126))(AmdCpmTableSmmProtocol, v10);
          LOBYTE(v13) = -120;
          v14 = (v11 << 8) + v12;
          v15 = (*(AmdCpmTableSmmProtocol + 126))(AmdCpmTableSmmProtocol, v13);
          LOBYTE(v16) = -121;
          v17 = (v14 << 8) + v15;
          v18 = (*(AmdCpmTableSmmProtocol + 126))(AmdCpmTableSmmProtocol, v16);
          ...
      }
      ...
  }
  ...
}  

As we can see from the pseudocode, there are many function calls from the interface, which has been located as follows:

// AMD_CPM_TABLE_SMM_PROTOCOL_GUID = af6efacf-7a13-45a3-b1a5-aafc061c4b79
gBS->LocateProtocol(&AMD_CPM_TABLE_SMM_PROTOCOL_GUID, 0, &gAmdCpmTableSmmProtocol);

The AMD_CPM_TABLE_SMM_PROTOCOL_GUID (af6efacf-7a13-45a3-b1a5-aafc061c4b79) protocol installation routine shown below (code from AmdCpmInitSmm module):

EfiSmmBase2Protocol->InSmm(EfiSmmBase2Protocol, &InSmram);
if ( !InSmram )
  return res;
Status = gSmst->SmmAllocatePool(EfiRuntimeServicesData, 0x410, &Buffer);
if ( Status >= 0 )
{
  Status = gBS->LocateProtocol(&AMD_CPM_TABLE_PROTOCOL_GUID, 0, &Interface);
  if ( Status >= 0 )
  {
    *Buffer = *Interface;
    *(Buffer + 1) = *(Interface + 1);
    InterfaceBuffer = Buffer;
    *(Buffer + 0x71) = sub_13A0;
    InterfaceBuffer[0x72] = &loc_13A8;
    InterfaceBuffer[0x78] = sub_14E4;
    InterfaceBuffer[0x79] = sub_14B4;
    sub_2DE4(Buffer);
    sub_3A00(Buffer);
    sub_5FC0(Buffer);
    Handle[0] = 0;
    return gBS->InstallProtocolInterface(Handle, &AMD_CPM_TABLE_SMM_PROTOCOL_GUID, EFI_NATIVE_INTERFACE, Buffer);
...

As we can see, the interface address (located with gBS->LocateProtocol()) is located inside SMRAM (since Buffer allocated with gSmst->SmmAllocatePool() function), but installed with gBS->InstallProtocolInterface() service.

The vulnerability cannot be exploited from the operating system. But an attacker capable of executing code in DXE could control af6efacf-7a13-45a3-b1a5-aafc061c4b79 protocol interface address.

In order to safely use the functions from this protocol interface within the SMI handler, this protocol must be installed with EFI_SMM_SYSTEM_TABLE2::SmmInstallProtocolInterface() and located with EFI_SMM_SYSTEM_TABLE2::SmmLocateProtocol().

Disclosure timeline

This vulnerability is subject to a 90 day disclosure deadline. After 90 days elapsed or a patch has been made broadly available (whichever is earlier), the vulnerability report will be made publicly available.

Acknowledgements

BINARLY efiXplorer team