In June 2024, a vulnerability was identified and resolved in the Linux kernel, specifically affecting the TI wl128x media driver. This issue, now recorded as CVE-2024-56700, involves an atomicity violation in the fmc_send_cmd() function. If you're running kernel versions with the affected drivers/media/radio/wl128x/fmdrv_common.c, you could be at risk for a potential kernel crash due to a classic race condition leading to a null pointer dereference.

This post will break down the bug in simple language, show code snippets of the problematic logic and the fix, and go step-by-step through how an attacker might exploit it or how a user would trigger it by accident. We’ll also look at tools and references for further learning.

What Is the Vulnerability?

Atomicity violations happen when a piece of data is checked and then used, but between the check and the use, some other thread changes the data. In this bug, the data is the fmdev->resp_skb pointer inside the fmc_send_cmd() function.

Here’s a simplified version of the problematic code flow

int fmc_send_cmd(struct fmdev *fmdev, /*...*/) {
    /* ... */
    if (fmdev->resp_skb != NULL) {       // (1) Check (Is not null?)
        skb = fmdev->resp_skb;           // (2) Use (Assign to local)
    }
    // ... later ...
    evt_hdr = (void *)skb->data;         // (3) Use for real
    
    /* ... */
}

What’s wrong?

- If someone else sets fmdev->resp_skb to NULL after the check, but before the assignment, skb could still be pointing to a released or wrong value.
- This could lead to a null pointer dereference at the point where the code tries to access skb->data (line 478 in the real file).

How Can It Be Exploited?

In practical terms, this is a tricky bug to exploit intentionally but could happen naturally on busy systems. Imagine two threads:

Another is cleaning up or resetting fmdev->resp_skb in parallel.

If the second thread zeroes the pointer between the check and the use, the first thread will work with a null or freed pointer, potentially crashing the kernel.

This flaw would mostly be hit by accident, but a motivated attacker with access to hardware and control over the device might try to trigger it for a Denial of Service (DoS).

Suppose you have code or bugs that could cause concurrent programming paths, such as

// Thread 1 (sending cmd)
fmc_send_cmd(fmdev, ...);

// Thread 2 (resetting or shutting down device)
fmdev->resp_skb = NULL;

If these happen at the right moment, Linux could panic.

The Fix: Keep It Locked

The fix, as implemented, is simple: Move the check for fmdev->resp_skb inside the locked section. That way, nobody can change it between checking it and using it. Here is the solution in code:

/* Old buggy code */
spin_lock(&fmdev->lock);
spin_unlock(&fmdev->lock);

if (fmdev->resp_skb != NULL) {
    skb = fmdev->resp_skb;
    //...
}

/* Corrected code */
spin_lock(&fmdev->lock);
if (fmdev->resp_skb != NULL) {
    skb = fmdev->resp_skb;
    //...
}
spin_unlock(&fmdev->lock);

Now, any time resp_skb is examined or used, the lock is held, so concurrent modification by other threads is impossible at this point.

Discovery: Static Analysis in Action

This bug was found by an experimental static analysis tool designed to uncover race conditions, atomicity bugs, and potential data races in Linux drivers. The tool checks for function pairs that can run at the same time and specifically looks for cases where data is accessed outside of required lock protections. It's a great example of research tools surfacing real security problems.

Commit fixing the bug:

Commit patch (LKML)

Kernel source file location:

drivers/media/radio/wl128x/fmdrv_common.c

CVE Record:

CVE-2024-56700 on cve.org (to be published)

Linux Kernel Media Subsystem:

Linux MediaWiki

For more about static analysis and race conditions

- The Linux Kernel documentation on Locking
- Coccinelle: Program matching and transformation for C

If you are a Linux distributor:

Make sure your kernel sources have this patch included, especially if you ship with wl128x radio hardware support.

General users:

There is low risk unless your device uses a wl128x radio chip, but all kernel privilege escalation and DoS bugs should be patched for safety.

Conclusion

CVE-2024-56700 is a concurrency flaw with possible denial-of-service impact. While niche, it’s a strong reminder of the importance of correct lock usage in kernel code. Even small windows between checking and using a pointer can cascade into real world reliability and security issues if left unchecked.

Stay safe and keep your kernels patched!

*This post is original and exclusive, referencing upstream sources and providing an easy-to-follow breakdown of CVE-2024-56700 and its fix.*

Timeline

Published on: 12/28/2024 10:15:17 UTC
Last modified on: 05/04/2025 10:02:47 UTC