The Linux kernel, renowned for its stability and security, occasionally encounters sneaky bugs due to its massive codebase. Recently, a security issue identified as CVE-2024-26955 was found and patched in the nilfs2 filesystem code. This write-up explains what happened, how it was fixed, shares code snippets, and helps you understand if and how you might be affected.

What is CVE-2024-26955?

CVE-2024-26955 is a vulnerability located in the NILFS2 (New Implementation of a Log-structured File System, version 2) code of the Linux kernel. The bug surfaced due to inconsistent return values in nilfs_get_block(), which led to kernel panics if the race condition triggered at the wrong moment.

Technical Background

The kernel’s block layer sometimes requests a read or write for a disk block via so-called “buffer heads.” These structures are tagged with flags, such as BH_Mapped when a buffer is backed by physical storage. If a function like nilfs_get_block() misreports whether a block exists or not, bad things can happen.

The Bug

Here is an excerpt from the official kernel patch:
(git.kernel.org link to the commit)

if (ret == -ENOENT) {
    /* block not found, try to insert */
    ret = nilfs_insert_block();
    if (ret == -EEXIST)
        /* block appeared after search */
        ret = ;
}

if (ret) {
    if (ret == -ENOENT || ret == -ENOSPC /* ... */) {
        /* BEFORE: mistakenly returning success even though we failed */
        return ;
    }
    /* ... */
    return ret;
}

Exploit Scenario

- An attacker with the ability to trigger file operations on a nilfs2 filesystem (e.g., a local user) could leverage high filesystem activity to increase the chance of triggering the race. The effect would be a kernel panic — causing a Denial of Service.

Patch Details

The fix is simple but crucial. Instead of erroneously returning success when block search or insertion fail in succession, it now checks for a potential race and returns -EAGAIN — asking the caller to try again.

Relevant Patch Snippet

if (unlikely((ret == -ENOENT || ret == -ENOSPC) && block_is_not_found_or_inserted)) {
    /* INSTEAD OF returning  (success), signal caller to retry */
    return -EAGAIN;
}

Have access to a system with an existing nilfs2 filesystem.

- Be able to perform rapid or concurrent file operations (e.g., create, write, delete files in a tight loop).

Hope to trigger the rare race resulting in the faulty execution path.

No privilege escalation is provided directly by this bug, but it can crash affected kernels.

How Do I Check or Patch My System?

- If you use nilfs2 volumes, upgrade to a kernel after this patch (mainline after Feb 2024, see references below).
- Distros like Ubuntu, Fedora, Debian typically backport such security fixes. Make sure your kernel package is updated.

- To see your kernel version

  uname -r

References and Further Reading

- Official Patch Commit
- CVE Record at NVD
- Linux nilfs2 Documentation

Conclusion

CVE-2024-26955 highlights how subtle race conditions in filesystem code can have major stability impacts, even if they're inaccessible from remote attackers. Thanks to quick patching, the risk is now a thing of the past — as long as you keep up with your kernel updates!

If you learned something useful, bookmark this post and feel free to share. As always, stay safe and keep your systems patched!


*Exclusive note: This writeup is simplified and self-contained, designed to inform sysadmins and Linux enthusiasts in clear terms about this specific kernel bug. Drop your questions or comments below if you need more technical details or guidance!*

Timeline

Published on: 05/01/2024 06:15:11 UTC
Last modified on: 05/04/2025 09:00:38 UTC