[BRLY-2021-045]
SMM callout vulnerability in USBRT SMM driver on Dell devices (SMM arbitrary code execution)
BINARLY efiXplorer team

Summary

BINARLY efiXplorer team identified SMM callout on Dell platforms, which allows a attacker to access the System Management Mode and execute arbitrary code.

Vulnerability Information

  • BINARLY internal vulnerability identifier: BRLY-2021-045
  • CERT/CC assigned case number: VU#540990
  • Dell PSIRT assigned CVE identifier: CVE-2022-24421
  • 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 team

Device/Firmware File Name SHA256 (File PE32 section) File GUID
DELL Inspiron 15 Gaming 7567 USBRT 05011cd7c3b27b2e723ed88f7c4d0512ed1045c5265897aefba73fd6b8e7fece 04EAAAA1-29A1-11D7-8838-00500473D4EB

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 vulnerability exists in SW SMI handler located at offset 0x1BC8 in the driver. The handler is registered as follows:

result = (gSmst->SmmLocateProtocol)(&EFI_SMM_SW_DISPATCH2_PROTOCOL_GUID, 0, &EfiSmmSwDispatch2Protocol);
if ( result >= 0 )
{
  RegisterContext.SwSmiInputValue = 0x31;
  EfiSmmSwDispatch2Protocol->Register(EfiSmmSwDispatch2Protocol, SmiHandler, &RegisterContext, &DispatchHandle);
  ...
}

The pseudocode of the vulnerable handler is shown below:

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

  UsbData = gUsbData;
  Struct = *(gUsbData + 25824);
  if ( Struct )
  {
    *(gUsbData + 25824) = 0;
  }
  else
  {
    if ( (*(gUsbData + 8) & 0x10) != 0 )
      return 0;
    Struct = *(16 * MEMORY[0x40E] + 260);
    // check that the buffer does not overlap with SMRAM
    if ( BufferValidation(Struct, 47) < EFI_SUCCESS )
      return 0;
    *(UsbData + 26904) = 1;
  }
  if ( !Struct )
    return 0;
  FuncIndex = *Struct;
  if ( !*Struct )
    goto _Exec;
  if ( FuncIndex >= 32 && FuncIndex <= 56 )
  {
    FuncIndex -= 31;
_Exec:
    (gUsbApiTable[FuncIndex])(Struct);
    UsbData = gUsbData;
  }
  if ( !*(UsbData + 25824) )
    *(UsbData + 26904) = 0;
  return 0;
}

Struct value can be controlled by attacker with Ring 0 privileges. If FuncIndex == 15, than the function located at offset 0x30D8 will be called:

__int64 __fastcall UsbApiCoreProc(STRUCT *Struct)
{
  // [COLLAPSED LOCAL DECLARATIONS. PRESS KEYPAD CTRL-"+" TO EXPAND]

  result = Invoke(gCoreProcTable[Struct->SubfuncIndex], Struct->ParamBuffer, (Struct->ParamSizeCoreProc + 3) & ~3u);// No index validation
  Struct->RetVal = 0;
  *&Struct->ApiRetValCoreProc = result;
  return result;
}

The first argument to the Invoke function is a pointer to be retrieved from the structure pointed to by gUsbData. The pseudocode of the Invoke function is shown below:

__int64 __fastcall Invoke(__int64 (*Ptr)(void), _QWORD *ParamBuffer, unsigned int ParamSize)
{
  // [COLLAPSED LOCAL DECLARATIONS. PRESS KEYPAD CTRL-"+" TO EXPAND]

  v3 = ParamSize >> 3;
  if ( !v3 )
    return Ptr();
  v4 = v3 - 1;
  if ( !v4 )
    return (Ptr)(*ParamBuffer);
  v5 = v4 - 1;
  if ( !v5 )
    return (Ptr)(*ParamBuffer, ParamBuffer[1]);
  v6 = v5 - 1;
  if ( !v6 )
    return (Ptr)(*ParamBuffer, ParamBuffer[1], ParamBuffer[2]);
  v7 = v6 - 1;
  if ( !v7 )
    return (Ptr)(*ParamBuffer, ParamBuffer[1], ParamBuffer[2], ParamBuffer[3]);
  v8 = v7 - 1;
  if ( !v8 )
    return (Ptr)(*ParamBuffer, ParamBuffer[1], ParamBuffer[2], ParamBuffer[3], ParamBuffer[4]);
  v9 = v8 - 1;
  if ( !v9 )
    return (Ptr)(*ParamBuffer, ParamBuffer[1], ParamBuffer[2], ParamBuffer[3], ParamBuffer[4], ParamBuffer[5]);
  if ( v9 == 1 )
    return (Ptr)(
             *ParamBuffer,
             ParamBuffer[1],
             ParamBuffer[2],
             ParamBuffer[3],
             ParamBuffer[4],
             ParamBuffer[5],
             ParamBuffer[6]);
  return 0;
}

According to this pseudocode, if an attacker can control a pointer, then he can execute an arbitrary function and pass up to 7 parameters to it.

Let's see what gCoreProcTable looks like:

.text:0000000080000FC0 gCoreProcTable  dq offset Proc0         ; DATA XREF: UsbApiCoreProc+15↓o
.text:0000000080000FC8                 dq offset Proc1
.text:0000000080000FD0                 dq offset Proc2
.text:0000000080000FD8                 dq offset Proc3
.text:0000000080000FE0                 dq offset Proc4
.text:0000000080000FE8                 dq offset Proc5
.text:0000000080000FF0                 dq offset Proc6
.text:0000000080000FF8 ; EFI_GUID EFI_SMM_RUNTIME_SERVICES_TABLE_GUID
.text:0000000080000FF8 EFI_SMM_RUNTIME_SERVICES_TABLE_GUID dq 413E287F395C33FEh
.text:0000000080000FF8                                         ; DATA XREF: sub_80001664+78↓o
.text:0000000080000FF8                                         ; sub_8000191C+88↓o ...
.text:0000000080001000                 dq 3ED4E1C0888055A0h
.text:0000000080001008                 dq 0
.text:0000000080001010                 dq 0
.text:0000000080001018                 dq 0

Struct->SubfuncIndex is controlled by an attacker and can take values from 0 to 255. The expected size of the gCoreProcTable array is 7, so it should check that Struct->SubfuncIndex < 7 before calling Invoke function.

Since there is no check for Struct->SubfuncIndex value, if the value of Struct->SubfuncIndex equal 9 or 10 or 11, it which can lead to arbitrary code execution, since gCoreProcTable[9] = gCoreProcTable[10] = gCoreProcTable[11] = 0.

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.

Disclosure Activity Date
Dell PSIRT is notified 2021-12-08
Dell PSIRT confirmed reported issue 2022-02-25
Dell PSIRT assigned CVE number 2022-02-25
Dell PSIRT provide patch release 2022-02-25
BINARLY public disclosure date 2022-03-10

Acknowledgements

BINARLY efiXplorer team