[BRLY-2022-017] SMM callout vulnerability in SMM driver (SMM arbitrary code execution).

Summary

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

Vulnerability Information

• BINARLY internal vulnerability identifier: BRLY-2022-017
• Insyde PSIRT assigned CVE identifier: CVE-2022-36338
• FwHunt rule: BRLY-2022-017
• CVSS v3.1 7.5 High AV:L/AC:H/PR:H/UI:N/S:C/C:H/I:H/A:H

Affected HP 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 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

The vulnerable module registers a child SW SMI handler with GUID 2970687c-618c-4de5-b8f9-6c7576dca83d:

gSmst->SmiHandlerRegister)(SmiHandler, &UNKNOWN_PROTOCOL_2970687C_GUID, &Handle);

The handler is located at offset 0x19D0:

EFI_STATUS __fastcall SmiHandler(EFI_HANDLE DispatchHandle, const void *Context, void *CommBuffer, UINTN *CommBufferSize)
{
  if ( CommBuffer && CommBufferSize && !gExitBootServicesFlag2EfiEventLegacyBootFlag )
  {
    if ( !gUnknownProtocol74d936fa )
    {
      if (gEfiBootServices->LocateProtocol)(&UNKNOWN_PROTOCOL_74D936FA_GUID, 0, &gUnknownProtocol74d936fa) < 0 )
        return 0;

As we can see before EFI_EVENT_EXIT_BOOT_SERVICES is triggered, the handler will try to locate protocol 74d936fa-d8bd-4633-b64d-6424bdd23d24 using the EFI_BOOT_SERVICES which is located outside of SMRAM, hence this call could be hijacked by a possible attacker.

Despite the vulnerability cannot be exploited from the operating system, 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).

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