CVE-2022-33984 is a recently disclosed vulnerability that can be found in some firmware implementations. It primarily targets the SdMmcDevice software SMI handler, leveraging Direct Memory Access (DMA) operations to exploit a time-of-check to time-of-use (TOCTOU) weakness. This can lead to System Management RAM (SMRAM) corruption, which is serious because SMRAM holds critical code and data used by the system’s SMM (System Management Mode).

This post will explain the issue in plain American English, break down the attack scenario, include example code snipplets, and link to the best available resources for further understanding. All information here is exclusive, focused, and hand-written.

What Is the SdMmcDevice SMI Handler?

SMI (System Management Interrupt) handlers are special routines running at the deepest privilege level on your computer. The *SdMmcDevice SMI handler* is part of the firmware, handling SD/MMC storage device operations.

SMRAM is a tightly protected memory region only accessible in SMM, making it a juicy target for attackers who can break in.

Usually, access to SMM and SMRAM is tightly restricted. However, under the right (or wrong) circumstances, attackers can sneak in through loopholes. That’s what’s happening with CVE-2022-33984.

At its heart, this vulnerability happens because of two things

1. DMA (Direct Memory Access): Hardware devices can copy data directly to or from RAM, bypassing the CPU.
2. TOCTOU (Time-of-Check to Time-of-Use): There’s a gap between when data is checked and when it’s used — and attackers can change the data in between.

The SdMmcDevice software SMI handler accepts *input buffers* from normal RAM, but attackers can perform DMA operations to change (or corrupt) those buffers after they’ve been checked but before SMI code actually uses them.

When the SMI handler copies this corrupted data into SMRAM, they’re essentially letting malware into one of the most privileged pieces of system memory.

Trigger the SMI handler — the handler checks the buffer (thinks it’s safe).

3. Quickly perform DMA — change the buffer to "malicious" data while the handler is still processing.

Handler copies the buffer to SMRAM, with the evil contents included.

This is your basic TOCTOU vulnerability, weaponized with DMA to bypass normal CPU-mediated security.

Example Code Snipplet

Here’s a schematic (not a “copy-paste” exploit!) to illustrate the kind of race condition involved:

// Pseudocode illustration only

void attacker_dma_overwrite(void* buffer, size_t size, void* malicious_data) {
    // Using a DMA engine or compromised device driver to overwrite RAM
    
    // Wait for the SMI handler to start and check the buffer
    wait_for_smi_check(buffer);
    
    // Rapidly overwrite the buffer before it's copied into SMRAM
    dma_write(buffer, malicious_data, size); 
}

// How the vulnerable SMI handler works
EFI_STATUS SMI_Handler(SMI_INPUT_BUFFER* buffer)
{
    if (!validate_input(buffer)) {
        return EFI_INVALID_PARAMETER;
    }
    // Vulnerable TOCTOU: buffer can be changed via DMA here!
    CopyMem((VOID*)SMRAM_address, buffer, buffer->Size);
    return EFI_SUCCESS;
}

In practice, the hard part is lining up the timing with a race. But with DMA hardware, it’s *very* feasible, especially for attackers with physical access or kernel-level malware.

Real-World Implications

- Once SMRAM is corrupted: Malicious code sitting in SMM can subvert system security, hide rootkits, or bypass virtualization/sandboxing.

Which Systems Are Affected?

- Vulnerability is in the *InsydeH2O* firmware (reference: SA-2022054).

How Was It Found?

This was discovered by Insyde engineering, following a general description provided by Intel’s iSTARE group.

Original References

- InsydeH2O Advisory SA-2022054

Additional technical background on the issue

- Intel® SMM Security & Firmware Attacks (Black Hat)
- DMA Attacks Explained (Eclypsium Blog)

Mitigation: How to Protect Yourself

- Update your firmware (BIOS/UEFI) to the latest version provided by your device vendor.
- Disable external DMA ports (Thunderbolt, FireWire, or unused PCIe slots) in BIOS if you don’t need them.
- Enable IOMMU/VT-d in BIOS settings — it helps restrict DMA access.
- Only use trusted expansion/peripheral devices.

Conclusion

CVE-2022-33984 underlines how complex and deep-rooted firmware security issues can be. A crafty attacker with DMA access can combine it with a simple TOCTOU vulnerability and open the gates to SMRAM compromise and full-system takeover.

If you’re responsible for managing PCs, workstations, or servers — check your firmware status and patch regularly. Firmware-level vulnerabilities aren’t hypothetical anymore.

- Insyde SA-2022054 Security Advisory

If you have any questions or need help checking your device, drop a comment!

Timeline

Published on: 11/15/2022 00:15:00 UTC
Last modified on: 02/14/2023 12:15:00 UTC