[BRLY-2022-022] SMM callout vulnerability in SMM driver (SMM arbitrary code execution).

Summary

Binarly REsearch Team identified a SMM callout, which allows an attacker to access the System Management Mode and execute arbitrary code.

Vulnerability Information

• BINARLY internal vulnerability identifier: BRLY-2022-022
• Insyde PSIRT assigned CVE identifier: CVE-2022-35408
• FwHunt rule: BRLY-2022-022
• CVSS v3.1 7.5 High AV:L/AC:H/PR:H/UI:N/S:C/C:H/I:H/A:H

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

Vulnerability description

Vulnerability exists in the USB System Management Interrupt (USBSMI) handler located at offset 0x13F4 in the module UsbLegacyControlSmm.efi.

The handler is registered as shown below:

RegisterContext.Type = UsbLegacy;
RegisterContext.Device = &gDevicePath;
if ( (EfiSmmUsbDispatch2Protocol->Register(EfiSmmUsbDispatch2Protocol, UsbSmiHandler, &RegisterContext, &Handle) & 0x8000000000000000) != 0 ) return EFI_UNSUPPORTED;

The pseudocode of USB SMI handler is shown below:

__int64 __fastcall UsbSmiHandler(
        EFI_HANDLE DispatchHandle,
        const void *Context,
        void *CommBuffer,
        UINTN *CommBufferSize)
{
  // [COLLAPSED LOCAL DECLARATIONS]

  result = 0;
  if ( (MEMORY[0xC00F8094] & 0xF00) != 0 )
    result = (MEMORY[0xC00F8094] >> 8) & 0xF;
  v5 = UsbLegacyProtocolFunc1;
  if ( UsbLegacyProtocolFunc1 )
    result = UsbLegacyProtocolFunc1(result, UsbLegacyProtocolFunc2);
  if ( !gNotTheFirstTime )
  {
    OldTpl = (gBS->RaiseTPL)(31);
    (gBS->RestoreTPL)(OldTpl);
    if ( OldTpl == 31 )
      (gBS->RestoreTPL)(8);
    gNotTheFirstTime = 1;
    if ( (gBS->LocateHandleBuffer(ByProtocol, &ProprietaryProtocol, 0, &NoHandles, &Buffer) & 0x8000000000000000u) == 0 )
    {
      v7 = 0;
      if ( NoHandles )
      {
        while ( (gBS->HandleProtocol(Buffer[v7], &ProprietaryProtocol, &Interface) & 0x8000000000000000u) != 0
             || *(Interface + 24) )
        {
          if ( ++v7 >= NoHandles )
            goto _Exit;
        }
        UsbLegacyProtocolFunc1 = *(Interface + 1);
        UsbLegacyProtocolFunc2 = *(Interface + 2);
      }
    }
_Exit:
    result = gBS->FreePool(Buffer);
    if ( OldTpl == 31 )
      return (gBS->RaiseTPL)(31);
  }
  return result;
}

From the pseudocode, we can see that the handler uses services from EFI_BOOT_SERVICES table (if the handler works for the first time according to the gNotTheFirstTime flag).

Thus, a potential attacker could overwrite the functions pointers in the EFI_BOOT_SERVICES table before the USB SMI handler triggered, which would lead to arbitrary code execution in the SMM.

The vulnerability cannot be exploited from the operating system. However, using EFI_BOOT_SERVICES and EFI_RUNTIME_SERVICES is unsafe inside a code intended to run in SMM (from SMRAM) because an attacker capable of executing code in DXE phase could exploit this vulnerability to escalate privileges to SMM (ring -2).This can be achieved even after EfiSmmReadyToLock event since no gSmst->InSmram() check is applied.

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