In June 2024, a new Linux kernel security issue was reported and fixed: CVE-2024-42081. This bug affected the drm/xe/xe_devcoredump component, dealing specifically with a missing NULL check that could have led to a crash or, in rare cases, insecure memory writes. This post will walk you through what this bug means, what changed, and how it could have been exploited.

What is CVE-2024-42081?

The vulnerability was found in the Intel XE graphics driver, specifically in the core dump handler code (xe_devcoredump). This kernel code is supposed to take a "snapshot" (a coredump) when something bad happens in the GPU driver. However, the code assigned pointers from a potentially NULL coredump variable without checking if it really exists. This is a classic NULL pointer dereference, which can crash the kernel or be leveraged under certain conditions.

Why is this important?
Kernel crashes cause system instability. In some cases, attackers can use such bugs for privilege escalation or information disclosure.

Take a look at what the buggy code could look like (simplified)

struct xe_devcoredump_snapshot *snapshot;
struct xe_device *dev;

snapshot = coredump->snapshot;
dev = coredump->dev;

// ... Use snapshot and dev, but what if coredump is NULL?

Problem: If coredump is NULL, then accessing coredump->snapshot or coredump->dev will crash the kernel!

The solution is to check for NULL _before_ dereferencing

struct xe_devcoredump_snapshot *snapshot = NULL;
struct xe_device *dev = NULL;

if (coredump) {
    snapshot = coredump->snapshot;
    dev = coredump->dev;
}

// ... Only use snapshot and dev if they aren't NULL

Result: Much safer, avoids NULL pointer dereference.

NULL pointer bugs in the kernel are risky

- Crash: A regular user could trigger subsystem bugs by doing unexpected things with the GPU, causing a system crash (DoS).
- Potential Exploit: Sometimes, attackers use kernel NULL pointers in combination with other techniques to execute arbitrary code or leak info.

This bug is primarily a Denial of Service risk. Here’s how an attacker might exploit it

1. Trigger a Devcore Dump: A regular user could perform operations that intentionally crash the GPU subsystem or stress the driver to hit this code.
2. Cause a NULL Pointer Access: If the coredump object is missing or failed to allocate, the kernel dereferences a NULL pointer.

Proof of Concept (Pseudo)

// User program triggers a GPU fault leading to devcoredump logic
// With system conditions causing coredump==NULL (resource exhaustion, etc)
// Kernel oops (crash) follows.

- Linux Kernel Patch for CVE-2024-42081 (lore.kernel.org)
- CVE-2024-42081 entry at cve.org
- Linux Kernel Bugzilla Entry (if publicized)
- Intel Graphics Driver Git

Update your kernel. Major Linux distros are now rolling out the fix.

- Monitor unusual GPU behavior. If you run graphics workloads or servers with active users, patch soon.
- Security teams: Consider blocking access to /dev/dri for untrusted users until the fix is applied.


Summary:
CVE-2024-42081 is a Linux kernel bug in the Intel XE driver that could have crashed your kernel if a GPU error occurred at the wrong time. The fix is a simple but critical NULL pointer check. Upgrade your kernel to stay protected.

Sources

- Original Kernel Patch and Discussion
- CVE Details

_This explanation is simplified and exclusive for those who want to understand kernel security in plain English!_

Timeline

Published on: 07/29/2024 16:15:07 UTC
Last modified on: 07/30/2024 18:57:21 UTC