Firmware security is a high-stakes game where any vulnerability can have serious consequences. In 2022, CVE-2022-33907 highlighted how a seemingly minor piece of UEFI firmware can be a launchpad for system compromise. This long-form post explains how the IdeBusDxe driver’s interaction with DMA-capable devices allowed for a classic race condition—TOCTOU (Time-Of-Check to Time-Of-Use)—that could corrupt the SMRAM and pave the way for attackers to gain ultimate privilege over a system. We break down what this means in practical terms, show sample exploit tactics, and provide links and code snippets for your understanding—all simplified for general technical audiences.

What’s the Deal with CVE-2022-33907?

First reported by Insyde Software and Intel’s iSTARE group, CVE-2022-33907 is a vulnerability in the IdeBusDxe driver found in Insyde’s UEFI firmware. The driver handles IDE bus interactions and supports communication with storage devices—but it’s also susceptible to Direct Memory Access (DMA) attacks targeting input buffers used by the software System Management Interrupt (SMI) handler.

Here’s the core problem: DMA transactions can target input buffers before the SMI handler processes them. A malicious device could change the content of an input buffer just after a check occurs but before it is used—a textbook TOCTOU (Time-of-Check to Time-of-Use) problem. Because the SMI handler has special privileges, corrupting this process can lead to unauthorized code execution inside SMRAM (System Management RAM), the “secure world” for firmware.

Why Is This Dangerous?

System Management Mode (SMM) runs at the highest privilege and is isolated from the main OS. SMRAM is supposed to be strictly protected. If an attacker corrupts it, they can:

Change firmware behavior

- Read/Write secrets (like keys)

Set Up a Malicious DMA Device:

The attacker controls a device capable of initiating DMA transactions. This could be an evil PCIe device or a compromised machine with DMA access.

Time-Of-Check: The SMI handler or firmware checks, validates, or uses the buffer pointer.

- Time-Of-Use: After the check, but before use, the malicious device changes the actual contents of the buffer via DMA.

Below is a simplified pseudocode of the problematic pattern commonly seen in firmware code

// Vulnerable SMI handler (simplified)
void SmiHandler(SMI_INPUT* InputBuffer) {
    // Step 1: Check/Validate contents
    if (InputBuffer->Command == ALLOWED_CMD) {
        // Step 2: Use buffer to perform a privileged action
        HandleCommand(InputBuffer->Data); // TOCTOU window: DMA can modify InputBuffer->Data
    }
}

Attack: The DMA device modifies InputBuffer->Data right after the if-statement but before HandleCommand().  
Result: The handler acts on malicious data.

What Is SMRAM and SMM?

- SMRAM: System Management RAM, a dedicated memory region used exclusively by SMM (System Management Mode).

SMM code: runs at a privilege level above the OS and hypervisor.

If SMM is compromised, the attacker can control nearly everything on the machine—including hiding malware from the operating system and firmware checks.

Exploit Example Overview

Here’s a conceptual outline—not a ready-to-weaponize proof, but enough to show how someone might exploit the bug.

Requirements:  
- Attacker can connect (or already control) a DMA-capable PCIe device (think external Thunderbolt interface or malicious PCIe card)

IdeBusDxe with vulnerable SMI handler is in use (pre-patch)

Steps:  
1. Scan system memory to identify SMI handler’s buffer (by sniffing for known patterns or trying device-specific calls).

Initiate an SMI event (e.g., issue an IDE request that triggers SMI).

3. While SMI is processing input, use DMA to overwrite the input buffer to inject malicious payload targeting SMM memory operations.

Payload is executed in SMM context, corrupting SMRAM or seizing SMM control.

---

Proof-of-Concept DMA Code (pseudo-Python)

import pcie_dma_lib

# 1. Locate SMI buffer -- in reality, requires OS or hardware knowledge
smi_buffer_addr = x1ffef000  # Hypothetical address

# 2. Wait for SMI trigger (out of scope, but could be polled)
wait_for_smi_event()

# 3. Overwrite buffer with malicious content
malicious_payload = b'\x90' * 256  # NOP sled as placeholder
pcie_dma_lib.write_mem(smi_buffer_addr, malicious_payload)


*This shows the DMA device overwriting the buffer in a race window.*

Kernel 5.4: 05.44.25

(Patch info: Insyde Security Pledge - SA-2022049)

Copy input buffers into SMRAM before use to prevent DMA-modifiable data.

- Lock input buffers against DMA upon SMI handling (e.g., use IOMMU/equivalent).

Real-World Impact

Attackers typically need some level of physical or privileged access—via a PCIe device, Thunderbolt port, or other DMA channels. But, as research into "Thunderspy" and other DMA attacks shows, such attacks are plausible in certain high-value targets (corporate, government, etc.).

Vulnerabilities in SMI and SMRAM have historically been a favorite of advanced persistent threats (APTs) because firmware-level persistence is devilishly difficult to clean up.

References & Further Reading

- Insyde Advisory: https://www.insyde.com/security-pledge/SA-2022049
- UEFI Security: https://uefi.org/security
- On SMM and DMA attacks: Black Hat USA 2019: Attacking UEFI firmware via DMA
- Intel Platform Security: https://www.intel.com/content/www/us/en/security-center/default.html

Conclusion

CVE-2022-33907 is a classic demonstration of how old attack patterns—like race conditions and DMA shenanigans—still haunt even the newest hardware. Updating UEFI firmware, auditing for SMI handler safety, and ensuring DMA protections are always enabled are the best defenses.

Timeline

Published on: 11/14/2022 23:15:00 UTC
Last modified on: 02/14/2023 12:15:00 UTC