Summary

BINARLY efiXplorer team has discovered a SMM callout vulnerability on multiple HP devices allowing a possible attacker to hijack execution flow of a code running in System Management Mode. Exploiting this issue could lead to escalating privileges to SMM.

Vulnerability Information

• BINARLY internal vulnerability identifier: BRLY-2021-041
• HP PSIRT assigned CVE identifier: CVE-2022-23933
• CERT/CC assigned case number: VU#683814
• CVSS v3.1 8.2 High AV:L/AC:L/PR:H/UI:N/S:C/C:H/I:H/A:H

Affected HP firmwares with confirmed impact by Binarly team

Device/Firmware	File Name	SHA256 (File PE32 section)	File GUID
Device / firmware version: 02.05.01 Rev.A	0216	af877a766e3364252ce75dfb8288b2e3e3df33fe0fa9b090f5fcc9ce8c57b6fe	6C9A8087-B68E-4F3D-B87D-A76C829C7D3E

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 the 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 vulnerability exists in the child SW SMI handler registered with GUID 3894c800-a50a-4cb2-afbc-9f0118377412 and located at offset 0x1988 in the driver.

The pseudocode for this handler is shown below:

EFI_STATUS __fastcall SmiHandler_1988(
        EFI_HANDLE DispatchHandle,
        const void *Context,
        void *CommBuffer,
        UINTN *CommBufferSize)
{
  // [COLLAPSED LOCAL DECLARATIONS. PRESS KEYPAD CTRL-"+" TO EXPAND]

  if ( CommBuffer )
  {
    if ( CommBufferSize )
    {
      if ( *(_DWORD *)CommBuffer == 'DFCH'
        && !gBS->AllocatePool(EfiBootServicesData, 175 * BlocksNum, (void **)CommBuffer + 2) )
      {
        DstBuffer = *((_QWORD *)CommBuffer + 2);
        Size = 175 * BlocksNum;
        *((_QWORD *)CommBuffer + 1) = BlocksNum;
        if ( DstBuffer )
        {
          if ( Size && gSrcBuffer && DstBuffer != gSrcBuffer )
            CopyMem(DstBuffer, (const __m128i *)gSrcBuffer, Size);
        }
      }
    }
  }
  return 0;
}

As we can see from this pseudocode, the AllocatePool service from the EFI_BOOT_SERVICES table is used.

Usage of EFI_BOOT_SERVICES and EFI_RUNTIME_SERVICES is unsafe inside a code intended to run in the SMM (from SMRAM), especially in SMI handlers, because an attacker capable of executing code in DXE phase could exploit this vulnerability to escalate privileges to SMM (ring -2).

To exploit this vulnerability it is enough to:

1. Find EFI_BOOT_SERVICES table in system memory.
2. Overwrite AllocatePool() service pointer in it with the shellcode address.
3. Trigger the SW SMI Handler (SW SMI number and pointer to Communication Buffer are specified in UEFI ACPI table) via 0xB2 IO port.

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
HP PSIRT is notified	2021-07-12
HP PSIRT confirmed reported issue	2021-08-09
HP PSIRT assigned CVE number	2021-08-19
CERT/CC created a case	2021-11-16
HP PSIRT provide patch release	2022-03-08
BINARLY public disclosure date	2022-03-08

Acknowledgements

BINARLY efiXplorer team