CVE-2024-26927 - Linux Kernel ASoC SOF Firmware Data Bounds Check Vulnerability Explained

A recently resolved vulnerability in the Linux kernel, tracked as CVE-2024-26927, has shed light on the importance of strict bounds checking when handling firmware data. This bug specifically affects the ASoC SOF (Sound Open Firmware) component, which handles audio processing on modern embedded devices. Though the issue was fixed promptly by kernel maintainers, it's important for developers and system administrators to understand the risk, the fix, and the potential for exploitation.

What is CVE-2024-26927?

CVE-2024-26927 is a bug in the ASoC SOF driver of the Linux kernel where the code failed to properly validate the sizes of firmware data headers when loading firmware. The function did not ensure that the subtraction head->full_size - head->header_size would not underflow (go negative and wrap around to a huge unsigned number), leading to possible memory overreads, kernel crashes, or potentially security issues if malicious firmware is introduced.

In short:
The kernel trusted the size values from firmware too much. If the firmware is malicious or corrupted, it could feed in unexpected values, making the kernel misbehave.

Technical Details & Vulnerable Code

The core of the issue is here:

The kernel was doing this (simplified C)

size_t data_size = head->full_size - head->header_size;

If head->header_size is greater than head->full_size, this subtraction underflows (because both are unsigned), and data_size becomes a massive number. Any logic that then uses data_size (like copying or parsing buffers) is at risk.

Smatch, an automatic code analyzer, flagged this during code review.

The Patch and Fix

The kernel maintainers added proper bounds checks before trusting these values.

Patched code

if (head->header_size > head->full_size) {
    dev_err(dev, "header_size > full_size in firmware header\n");
    return -EINVAL;
}
if (head->full_size > MAX_FIRMWARE_SIZE) {
    dev_err(dev, "full_size out of bounds in firmware header\n");
    return -EINVAL;
}
size_t data_size = head->full_size - head->header_size;

Exploit Potential

Who is at risk?
Anyone loading audio firmware via the SOF driver—this typically happens on some Intel-based laptops and other embedded devices.

How could an attacker exploit this?
To exploit this, an attacker would need to be able to load malicious firmware—either by tricking root/admin into loading their firmware, or by exploiting another vulnerability to escalate privileges. Then, by setting header_size to a large value and full_size to a smaller one, the attacker can cause the subtraction to underflow, letting them trick the kernel into parsing memory it shouldn't.

Consequences:
- Kernel panic/crash (Denial of Service).

Safe Version

Check your Linux kernel version and update!

Fix description: "ASoC: SOF: Add some bounds checking to firmware data"

- Mainline commit: link

Links and References

- Kernel Git Commit c8fdca040eeb297d54c9b5b9d83db9961e73b6b8
- CVE-2024-26927 entry at cve.org
- Linux Kernel SOF GitHub
- Smatch (static code checker)

Conclusion

While this issue is already fixed, CVE-2024-26927 is a great example of why kernel code must always be strict with untrusted input, even from firmware, which we often assume to be safe. Simple arithmetic checks can prevent serious bugs down the line. Keep your systems patched, audit your supply chains, and remember: don't trust unverified firmware!

Timeline

Published on: 04/28/2024 12:15:21 UTC
Last modified on: 08/02/2024 00:21:05 UTC