CVE-2023-52821 - Understanding the Linux Kernel drm/panel Null Pointer Dereference Vulnerability

In late 2023, a vulnerability tracked as CVE-2023-52821 was resolved in the Linux kernel. This security issue was found in the Direct Rendering Manager (DRM) panel subsystem, specifically affecting how the kernel handles display modes. The bug could trigger a NULL pointer dereference—a common but still dangerous type of flaw—that could cause a system crash or unexpected behavior. In this article, we will break down what happened, look at the vulnerable code, explain how it could be exploited, and show how the fix works.

What is the DRM Panel Subsystem?

The Direct Rendering Manager (DRM) subsystem in Linux is a crucial part of how graphics are handled. The DRM "panel" drivers manage communication with LCD screens and panels on embedded boards or laptops. They are responsible for configuring and managing display modes, resolutions, and other settings.

When a display connector’s supported resolutions are queried, the kernel driver duplicates a "mode" structure, which represents screen settings (like width, height, refresh rate). If there’s a bug in how these structures are handled, it can impact display functionality and, in more serious cases, overall system stability.

Details of the Vulnerability

The function in question, versatile_panel_get_modes, tries to duplicate a display mode using the helper function drm_mode_duplicate(). However, if for some reason this duplication fails (such as a memory allocation failure), the returned pointer is NULL.

Instead of checking for this NULL pointer, the original code assumed it always got a valid mode, leading to a NULL pointer dereference soon after. This is a type of programming bug where the code tries to access data through a pointer that is NULL, causing the kernel to crash (often called a kernel panic).

Let’s look at the code as it was before the patch (simplified for clarity)

struct drm_display_mode *mode;

mode = drm_mode_duplicate(connector->dev, &versatile_panel_mode);
// No NULL check!
drm_mode_set_name(mode);
drm_mode_probed_add(connector, mode);

Notice that after calling drm_mode_duplicate, the code skips checking if mode is NULL.

The Patch: How It Was Fixed

Developers closed the hole by adding a NULL check. If drm_mode_duplicate() fails and returns NULL, the function now quits early, avoiding the dangerous dereference.

Here’s how the fixed code looks

struct drm_display_mode *mode;

mode = drm_mode_duplicate(connector->dev, &versatile_panel_mode);
if (!mode)            // <- New check added
    return ;

drm_mode_set_name(mode);
drm_mode_probed_add(connector, mode);

Now, if mode is NULL, the function just returns zero, indicating that no modes were added.

Reference:
- Upstream kernel commit
- NVD entry for CVE-2023-52821

How Could This Be Exploited?

This flaw is a classic local denial of service (DoS) bug. A local user (or potentially a malicious userspace program) could trigger conditions causing drm_mode_duplicate() to fail (for example, by exhausting kernel memory). If the system attempts to access modes via the vulnerable function at that moment, it would dereference a NULL pointer and crash the kernel. This would take the affected system offline until it boots again.

As the attack requires either local access or the ability to interact with DRM devices, it is not a remote code execution or privilege escalation bug, but it could be used by attackers to force service outages or disruption.

Step-by-Step: Triggering the Issue (Proof of Concept)

This issue is hard to exploit directly from user-space unless the attacker has control over kernel memory allocations or a way to trigger DRM connector actions under low-memory conditions. However, in a lab setting, one might attempt:

Trigger display mode probing via user-space tools (like xrandr or by hotplugging displays).

3. With unlucky timing, the kernel could hit the NULL return from drm_mode_duplicate and trip over the missing check, leading to a kernel crash.

*Note: Exploiting this on a modern, production system is difficult, but the theoretical risk makes it important to fix in all kernels.*

If you manage Linux systems, especially with custom or embedded graphics hardware, make sure to

- Update to a kernel version containing the fix (mainline Linux 6.7 or with backported patches for supported LTS kernels).

Conclusion

CVE-2023-52821 may look like a small programming oversight, but even minor mistakes in kernel code can have real consequences. Simple checks—like verifying return values after memory allocations—are essential for reliability. Thanks to the quick work of upstream developers, this flaw was fixed before it could become an issue in the wild. Always keep your kernel up-to-date for security and stability.

Timeline

Published on: 05/21/2024 16:15:20 UTC
Last modified on: 05/24/2024 01:14:35 UTC