CVE-2024-27040 is a recently addressed vulnerability in the Linux kernel, specifically affecting the AMD display driver’s eDP (Embedded DisplayPort) replay feature. This bug could cause a system crash due to a NULL pointer dereference. Let’s break down what happened, how the bug works, and what it means for Linux users or anybody relying on AMD display hardware.

What is CVE-2024-27040?

In simple terms, this vulnerability happens when the AMD GPU driver, under certain conditions, tries to call a function inside a structure that is not initialized (i.e., it's actually a NULL pointer). When this happens, the kernel can crash, resulting in a denial-of-service (DoS). The problem lives in the edp_set_replay_allow_active() function in the drivers/gpu/drm/amd/display/dc/link/protocols/link_edp_panel_control.c part of the kernel source code.

Here’s a simplified rundown

- The code is supposed to interact with a "replay" structure related to power optimization in eDP (laptop display technology).
- The code does check if replay is NULL at first, but then *later* uses replay again without re-checking whether it’s still safe to do so.
- If replay is NULL at that later point, Linux will try to use a NULL pointer, quickly crashing the system or the video subsystem.

Here’s a simplified code snippet to show the bug

if (replay == NULL && force_static)
    return false;

// ... some code in between ...

if (link->replay_settings.replay_feature_enabled &&
    replay->funcs->replay_set_power_opt) {
    replay->funcs->replay_set_power_opt(replay, *power_opts, panel_inst);
    link->replay_settings.replay_power_opt_active = *power_opts;
}

If replay is NULL, then trying to do replay->funcs->replay_set_power_opt will cause a crash.

What Could an Attacker Do?

This bug is mostly a denial-of-service (DoS). If someone, possibly a local user or a malicious script, can trigger display reinitialization with this path and force the replay pointer to be NULL, they could crash the system kernel. There’s no known way to escalate privileges or execute code—just freeze or reset the system.

However, this is bad news in environments requiring high up-time like servers with attached AMD graphics, or multi-user laptops/desktops running Linux.

How Was It Fixed?

The fix is simple: always check if replay is NULL before trying to use it—improving the safety of the driver code.

Here’s how the patched code looks

if (replay && link->replay_settings.replay_feature_enabled &&
    replay->funcs->replay_set_power_opt) {
    replay->funcs->replay_set_power_opt(replay, *power_opts, panel_inst);
    link->replay_settings.replay_power_opt_active = *power_opts;
}

Now, the function will only call the next function if replay is not NULL. No more kernel crashes!

References and Further Reading

- Linux Kernel Commit Fix (official kernel.org)
- Smatch static checker project
- Linux AMDGPU driver mailing list
- NVD Record for CVE-2024-27040

Is There An Exploit?

There is no public exploit for this bug, and it would likely require direct access to the system and hardware. However, a simple proof-of-concept can be crafted by manipulating kernel modesetting or DRM-related operations to trigger the bug.

Example Pseudocode Exploit Path

> Warning: *Do not run untrusted code or scripts as root. Kernel bugs can lock up or damage your system.*

// This is NOT working exploit code, just a conceptual demonstration
void trigger_bug() {
    // Find or force replay pointer to NULL in the context
    // (In practice, this is hard to do directly from userspace!)
    // Ask the system to adjust eDP panel power options
    drmIoctl(drm_fd, EDP_SET_REPLAY_ALLOW_ACTIVE, ...);
    // If the kernel dereferences NULL, the machine may crash here.
}

In reality: This is a kernel-level flaw, only directly triggerable by processes with high privileges or by abusing flaws at the user-kernel interface in the DRM subsystem.

Who Should Worry?

- Anyone running mainline Linux 6.x kernels + AMD graphics hardware and eDP panels (most AMD laptops or similar devices).

Patch your kernel! Most major distributions have already backported the fix.

- Check your system: If you have an AMD GPU and run kernel 6.8 or earlier, make sure updates are applied.
- Userspace code is not affected if you’re not using AMD hardware/eDP displays.

Conclusion

CVE-2024-27040 is a clear example of how small mistakes (like failing to check if a pointer is NULL) can have big consequences in low-level code like the Linux kernel. If you’re running affected hardware, update as soon as possible. Keep an eye on kernel advisories and always stay updated.

For more technical details, read the original kernel patch and stay tuned for updates from your distribution.

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Timeline

Published on: 05/01/2024 13:15:49 UTC
Last modified on: 11/07/2024 17:35:15 UTC