Earlier this year, security researchers identified a subtle yet important vulnerability in the Nouveau driver of the Linux kernel. The bug, tracked as CVE-2024-56752, affected the gf100 GPU family support code. Specifically, the issue occurred due to a missing unlock operation on a mutex within the gf100_gr_chan_new() function, which could result in a kernel deadlock.

Below, we break down how the vulnerability worked, where it came from, and show you the relevant Linux kernel code that caused it. We’ll also walk you through how this bug could have been exploited, plus provide details about the patched fix. The content here is tailored for a technical audience, but explained using plain American English.

The Short Answer

In some error situations, the code failed to release a lock (mutex), meaning other kernel code or drivers waiting to use this resource could end up blocked forever. This creates risks for system stability and maybe availability (but no direct privilege escalation or data leak risk was reported).

The Longer Explanation

In the Nouveau driver module, whenever a new graphics channel is created (gf100_gr_chan_new()), it first acquires a mutex (gr->fecs.mutex). If a later function call (gf100_grctx_generate()) failed, instead of unlocking the mutex before returning the error, the code just bailed out—leaving the mutex still locked. Other kernel components trying to acquire that mutex would hang, causing either performance degradation or, in rare cases, a complete system freeze.

Take a look at the simplified vulnerable code (as it appeared before the patch)

// File: drivers/gpu/drm/nouveau/nvkm/engine/gr/gf100.c

int gf100_gr_chan_new(...) {
    ...
    mutex_lock(&gr->fecs.mutex);

    ret = gf100_grctx_generate(...);
    if (ret) {
        // ERROR! Missing unlock here
        return ret; 
    }

    mutex_unlock(&gr->fecs.mutex);
    ...
    return ;
}

Problem?
If gf100_grctx_generate fails (ret != ), the code returns immediately, and never calls mutex_unlock, so the locked section stays locked.

Patch & Resolution

The fix was straightforward: ensure the lock is always released before returning, even if there's an error.

Patched code

int gf100_gr_chan_new(...) {
    ...
    mutex_lock(&gr->fecs.mutex);

    ret = gf100_grctx_generate(...);
    if (ret) {
        mutex_unlock(&gr->fecs.mutex);   // <- FIX: Unlock before returning!
        return ret; 
    }

    mutex_unlock(&gr->fecs.mutex);
    ...
    return ;
}

Reference:
- Linux kernel commit 8e37d5c5c8

Exploit Details

While this is not a security vulnerability in the sense of arbitrary code execution, it is a Denial of Service (DoS) issue:

Attack Vector:

Since the bug sits in the GPU subsystem, to trigger this, an attacker would need to cause gf100_gr_chan_new() to fail after the mutex lock is acquired but before it is released. This could potentially be done by providing malformed input or causing resource exhaustion.

Denial of Service Result:

If successfully triggered, the mutex never unlocks. That means any future attempts to use the GPU by other users, graphics processes, or system tasks could hang indefinitely.

Potential Exploit:

A local attacker could write code which (via ioctl or other nouveau driver interfaces) triggers the error path. Although no privilege escalation occurs, they may be able to freeze the graphics subsystem for other users and cause a system hang or crash.

Original Patch:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=8e37d5c5c8a8e10d65c4da19a6044e5012ca4367

Nouveau Driver Codebase:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/gpu/drm/nouveau/nvkm/engine/gr/gf100.c

Nvidia GF100 architecture:

https://en.wikipedia.org/wiki/GeForce_400_series

Syzkaller bug report:

Linux Kernel Bugzilla Entry (example) (example link—actual bug number may vary)

Summary Table

| Item | Details |
|---------------------|------------------------------------------------------|
| CVE | CVE-2024-56752 |
| Component | Linux Kernel Nouveau driver, GF100 generation |
| Vulnerable Code | Missing mutex_unlock in error return pathway |
| Exploit Impact | Kernel hang, GPU subsystem DoS (no escalated privs) |
| Patched | Yes, released in mainline kernel |

How to Stay Safe

- Upgrade your kernel. Make sure your distro is running a kernel with this fix, especially if you use Nouveau with a GF100 or related Nvidia GPU.
- Monitor dmesg/logs for nouveau-related errors or GPU hangs.
- Report any bugs you find in graphics driver codepaths directly to kernel maintainers or through your distribution’s security channels.

Conclusion

CVE-2024-56752 is a great reminder that even small mistakes in kernel driver code can have huge consequences, locking up entire subsystems. Thanks to quick work by the kernel community, this bug was fixed before widespread exploitation, but it’s always smart to stay vigilant and updated.

If you’re interested in the technical details, check the links above and watch for lock-unlock patterns—missing one unlock can lock up your whole system!

Timeline

Published on: 12/29/2024 12:15:08 UTC
Last modified on: 01/06/2025 19:11:42 UTC