CVE-2024-43902 is a recent Linux kernel vulnerability patched in June 2024, affecting the AMD display subsystem. It was resolved upstream thanks to security researchers and automated static analysis, notably with Coverity. This long-read explains what led to the bug, how it could be exploited, and the details behind the fix – all in simple terms.

What is CVE-2024-43902?

The Linux kernel’s AMDGPU Direct Rendering Manager (DRM) display code had functions that could potentially receive NULL pointers as input. If these pointers weren’t checked for NULL before use, the kernel could dereference them—leading to a crash (kernel panic), system instability, or potentially opening up other vectors for local attackers.

Summary:
- Issue: Functions in drm/amd/display accepted pointers from function callers but didn’t check if they were NULL.
- Affected Component: Linux kernel, module: drivers/gpu/drm/amd/display

Let’s see a conceptual code snippet

// Vulnerable before patch
void amd_display_do_something(struct amdgpu_device *adev) {
    do_something_crucial(adev->dm);
}

// If adev->dm is NULL, the system crashes here:
void do_something_crucial(struct amdgpu_display_manager *dm) {
    // dereference dm...
}

Here, if adev->dm is NULL, the pointer is sent to do_something_crucial which tries to use it—this leads to a crash.

To exploit this, an attacker could

- Trigger specific code paths (perhaps via crafted system calls, or direct device node access) to cause the driver to work with uninitialized or incorrectly-initialized structures.

Consequences

- Denial of Service: Crashes the system (kernel panic). You’ll see logs mentioning segmentation faults in the AMD GPU kernel code.
- Local User Impact: A local user with limited privileges can crash the entire computer by abusing the flawed code path.

This vulnerability is not *directly* a privilege escalation, but in shared desktop/server environments, DoS is considered a serious threat.

The Fix

In June 2024, AMD engineers submitted a patch upstream (committed by Alex Deucher) that addresses this by checking for NULL before using pointers.

Patch Example

// Fixed version
void amd_display_do_something(struct amdgpu_device *adev) {
    if (adev->dm == NULL) {
        // Log an error, avoid crash
        return;
    }
    do_something_crucial(adev->dm);
}

Or, more generally, code patterns like

if (variable != NULL)
    call_function(variable);

This pattern avoids dereferencing NULL pointers. The patch fixed three such instances.

Commit Link:
- drm/amd/display: Add null checker before passing variables (kernel.org)

Relevant code diff (excerpt)

+    if (adev->dm == NULL)
+        return;
     do_something_crucial(adev->dm);

Who is Vulnerable?

- Users running Linux distributions with the AMDGPU DRM driver, especially on kernels prior to the fix commit.

Check Your Kernel

Look for mention of “CVE-2024-43902” or the commit hash above in your distro’s kernel changelog.

Recommendation

Update: Always upgrade to the latest kernel provided by your Linux distro. This vulnerability has been patched in the upstream kernel and should be quickly adopted by major distributions.

Original References

- AMD Patch: drm/amd/display: Add null checker before passing variables
- Coverity Issue (coverage of kernel issues)
- CVE-2024-43902 Record (MITRE) *(available after CVE publication)*

Conclusion

CVE-2024-43902 is a classic demonstration of how missed null pointer checks can endanger system security, and how systematic static analysis tools like Coverity safeguard our ecosystems. The fix is simple yet crucial—a reminder that even basic code hygiene matters, especially in kernel space where the consequences of crashes are severe.

Advice:
Update your systems, watch for similar patterns in your code, and thank your distro maintainers for keeping up with patches like this!

Timeline

Published on: 08/26/2024 11:15:04 UTC
Last modified on: 08/27/2024 14:38:51 UTC