---

Overview

CVE-2024-57876 is a critical vulnerability found in the Linux kernel DRM (Direct Rendering Manager) subsystem, specifically in the DisplayPort Multi-Stream Transport (DP MST) message handling. This flaw could lead to a race condition, causing memory corruption when the MST topology is removed asynchronously during a sideband message reception.

In this long read, we’ll break down what this vulnerability is, how it was found, how an attacker could potentially exploit it, and how it was fixed. We’ll also look at relevant code snippets to help you understand the issue, and provide references for further reading.

What is DRM DP MST?

The Linux kernel’s DRM subsystem manages display hardware. DP MST allows a single DisplayPort connection to carry multiple independent video streams (like daisy-chained monitors). The kernel tracks this using a structure called drm_dp_mst_topology_mgr, which manages state for sending and receiving control messages (so-called *"sideband messages"*).

Where's the Bug?

The affected code revolves around handling MST sideband messages received from attached monitors. Two states – up_req_recv and down_rep_recv – keep track of incoming message status. If a MST topology is removed (imagine unplugging monitors) while a message is being processed on another thread, these states can be cleared without coordination.

drm_dp_mst_topology_mgr_set_mst(false) – Removes the topology and clears state from thread A.

- drm_dp_mst_handle_down_rep() or drm_dp_mst_handle_up_req() – Reads and parses the state (possibly at the same time from thread B), but without holding a lock.

If thread A resets state in the middle of thread B’s parsing, this leads to inconsistent state and memory corruption.

The Race Condition

The key is missing proper locking. The parsing thread can read invalid/outdated pointers while the other thread is resetting state.

A simplified code snippet for illustration

// Thread A: removing the topology, resets reception state
void drm_dp_mst_topology_mgr_set_mst(struct drm_dp_mst_topology_mgr *mgr, bool mst_state) {
    if (!mst_state) {
        mgr->up_req_recv = false;
        mgr->down_rep_recv = false;
        // ... topology removal ...
    }
}

// Thread B: receiving a reply, reads state
void drm_dp_mst_handle_down_rep(struct drm_dp_mst_topology_mgr *mgr) {
    // No locking here...
    if (mgr->down_rep_recv) {
        // Parse message, possibly using stale data!
    }
}

Attack Surface

While this bug is mostly a local issue (i.e., someone with access to the physical machine or compromised display devices), a malicious device or process could intentionally trigger the race by spamming message traffic while hotplugging displays.

Exploit Possibility

If an attacker can control the timing of MST device disconnects and sideband messages, the race could be abused to cause memory corruption, likely leading to a kernel crash (DoS), or in rare cases, possible privilege escalation. Real-world exploitation is difficult but not impossible, especially for an attacker able to physically interact with the system.

Exploit Steps Example:

Send a long or malformed sideband message to the kernel.

2. At the precise moment the kernel is parsing the message, rapidly remove the MST topology (disconnecting the DP device).

Patch Summary

The fix was to reset the reception state before starting message parsing, eliminating the window for the race. This is a minimal patch to make it easily backportable to stable kernels, instead of introducing a lot of locking changes.

Fixed Code Snippet

void drm_dp_mst_handle_down_rep(struct drm_dp_mst_topology_mgr *mgr) {
    // Reset message reception state BEFORE reading or parsing
    reset_message_reception_state_if_needed(mgr); // new helper

    // Now proceed safely with message parsing
    if (mgr->down_rep_recv) {
        // Parse message safely...
    }
}

By resetting the state up front, parsing never happens with invalid pointers even if another thread removes the topology.

Source Commit

The upstream fix is described here:
- drm/dp_mst: Fix resetting msg rx state after topology removal

References

- CVE-2024-57876 at NVD
- Linux kernel commit bd2fccac61b4
- drm_dp_mst_topology_mgr documentation
- Linux Kernel DRM subsystem

Conclusion

CVE-2024-57876 is another reminder that concurrency bugs in the Linux kernel, especially around hardware hotplug events, can have serious consequences for system stability and security. The solution here was a straightforward but vital cleanup, making sure message state is consistently managed before any critical parsing happens.

What You Should Do

- Update your kernel to the latest version that includes this fix (patches are being backported to all maintained stable branches).

If you're a device or distro maintainer, review your update pipelines to ensure quick rollouts.

For advanced users and kernel developers, keep a close eye on MST state changes, and ensure all concurrent accesses to shared structures are properly locked or clearly serialized.

Timeline

Published on: 01/11/2025 15:15:07 UTC
Last modified on: 05/04/2025 13:01:27 UTC