The Linux kernel is the core piece of software powering most servers, cloud infrastructure, and embedded devices around the world. It’s designed for both performance and security, but sometimes bugs slip through even in important subsystems. One such bug, now patched, is tracked as CVE-2024-39481, which could crash systems using the media framework. Below, we break down the vulnerability in plain language, show the affected code, give steps to exploit, and share how to fix it.

What Is CVE-2024-39481?

CVE-2024-39481 is a vulnerability in the Linux kernel's media controller (MC) framework.

When: Reported and patched in June 2024

- Impact: Any user or attacker able to trigger complex media pipelines could cause a kernel crash (denial of service), particularly by creating certain invalid ancillary links

Technical Overview: The Graph Walk Problem

The Linux kernel’s media subsystem uses a “graph” to represent the connections (links) between different media entities (like cameras, codecs, etc). These links can carry data, or serve other, “ancillary” (supplementary) purposes.

The vulnerable code tried to walk or traverse *all* links, regardless of their type, as part of the media_pipeline_start() routine:

Here’s a simplified version of the buggy code

// Example from media_pipeline_start()
list_for_each_entry(link, &entity->links, list) {
    // THIS IS FAULTY: follows all types of links!
    next = media_entity_other_entity(link, entity);
    if (!next)
        continue;
    // walk to the next entity...
}

The Crash Scenario

If a complex user application or crafted input created an “ancillary” link, and media_pipeline_start() tried to walk it (because it didn't check the link type), the kernel would follow an invalid pointer, leading to a crash.

How Was It Fixed?

The patch stops the traversal at *only* data links (MEDIA_LNK_FL_DATA_LINK). This avoids following harmful or irrelevant links.

Fixed code

// Safe: only follow data links!
list_for_each_entry(link, &entity->links, list) {
    if (!(link->flags & MEDIA_LNK_FL_DATA_LINK))
        continue; // Skip if not a data link
    next = media_entity_other_entity(link, entity);
    if (!next)
        continue;
    // walk to the next entity...
}

Commit Reference:
- Upstream patch

Proof-of-Concept Exploit

While exploitability is limited (requires CAP_SYS_ADMIN or equivalent privileges, or use by an authorized user/process), a minimal proof-of-concept would look like this:

1. Setup: Use media-ctl, or write a user-space program to create a media graph with an ancillary link between pads.

Pseudo steps

# 1. Find a media device (example: /dev/media)
# 2. Use v4l2-ctl or a custom user space app to create an ancillary link
# 3. Trigger media_pipeline_start (e.g., start video capture)
# Result: Kernel crash before patch

> If you do not have deep kernel knowledge, you likely won’t hit this bug by accident—but attackers with access could use it for denial of service.

Are you vulnerable?

- Kernel versions before the June 2024 patch with a linux-media controller compiled in, and using complex/non-default media graphs

How to fix

- Upgrade to a kernel version with the patch (commit).
- Downstream distros (Ubuntu, Debian, Fedora, etc.) are rolling out updates in June/July 2024.

Workaround:
Unless you can un-load or blacklist the media controller kernel module, there is no quick workaround.

References

- Mainline Patch: git.kernel.org - media: mc: Fix graph walk in media_pipeline_start
- Linux Kernel CVE database
- Linux media subsystem documentation

Conclusion

CVE-2024-39481 is a kernel bug that could be abused by privileged users or processes to crash Linux systems using advanced media hardware. The fix is now available upstream; updating your kernel is the safest step. If you develop drivers or have custom device pipelines, double-check your usage.

Stay patched, and stay safe!

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Timeline

Published on: 07/05/2024 07:15:10 UTC
Last modified on: 07/15/2024 06:50:17 UTC