A vulnerability in the Linux kernel that affects the drm/v3d driver has now been resolved. This issue, identified as CVE-2025-21697, was caused by not setting the job pointer to NULL after the job's completion while using the drm/v3d driver. Consequently, the device's driver would prompt a warning message when unloading, as the job seemed to still be active. In this post, we will delve into the details of this vulnerability, review the original code snippets, look into the exploit particulars, and provide links to references to help better understand this issue.

How the vulnerability was discovered

The Linux kernel community saw that even after the successful completion of a job, the pointer associated with it was not being set to NULL. As a result, when attempting to unload the driver, the kernel would flag an alert, suggesting the job was still running.

Here is a code snippet from the Linux kernel illustrating the issue

static void v3d_job_done(struct v3d_job *job)
{
    struct v3d_dev *v3d = job->v3d;

    // ... code truncated for brevity ...

    if (job->bo)
        dma_fence_put(job->render_done_fence);

    kref_put(&job->refcount, v3d_job_free);
}

The following section is how the code was fixed to ensure that the job pointer is set to NULL after the job is completed:

static void v3d_job_done(struct v3d_job *job)
{
    struct v3d_dev *v3d = job->v3d;

    // ... code truncated for brevity ...

    if (job->bo)
        dma_fence_put(job->render_done_fence);

    kref_put(&job->refcount, v3d_job_free);
    job = NULL; // Fix: set job pointer to NULL after job completion
}

By adding the job = NULL; line, the job pointer is now set to NULL after the job is completed, preventing the warning message from appearing when unloading the driver.

Exploit Details

An attacker could leverage the vulnerability by using the module unloading operation as a part of the attack vector. When unloading the drm/v3d driver, the kernel could be manipulated into improper handling of the job, leading to an inconsistent system state or crashes.

However, the attack surface is limited due to the nature of the bug, and casual exploitation is not expected. While this vulnerability can be a part of a multi-stage attack plan, its standalone exploitation is unlikely.

Original References

1. Linux kernel - drm/v3d driver source code: https://github.com/torvalds/linux/blob/master/drivers/gpu/drm/v3d

2. Linux kernel mailing list discussions related to drm/v3d vulnerability fix: https://lkml.org/lkml/2022/2/11/1477

3. CVE-2025-21697 details and mitigation strategy: https://nvd.nist.gov/vuln/detail/CVE-2025-21697

Conclusion

The Linux kernel vulnerability CVE-2025-21697 has been resolved by addressing the improper handling of the job pointer in the drm/v3d driver. Although the exploit potential remains limited, it is essential to stay informed about such vulnerabilities to maintain the security and stability of systems. We recommend updating your Linux kernel to the latest version and regularly checking for updates regarding security fixes to safeguard against potential exploits.

Timeline

Published on: 02/12/2025 14:15:32 UTC
Last modified on: 02/14/2025 14:13:39 UTC