Overview:
A dangerous vulnerability was recently patched in the Linux kernel, specifically in the Ceph distributed file system subsystem. The issue—tracked as CVE-2024-53685—let any user or attacker create a simple denial of service (DoS) situation by triggering an endless loop inside the kernel. Read on as we unpack what went wrong, how it happens, and how the fix solves it. We’ll walk through the details using plain language and easy-to-follow code, with links to the original patch and further resources.

What is Ceph and Why Does It Matter Here?

Ceph is an open-source distributed storage platform widely used with Linux, capable of offering massive, fault-tolerant, and easily scalable file storage.

At the heart of Ceph's connection to Linux is a kernel driver: it allows Linux systems to mount Ceph storage like a regular filesystem. Inside this driver is a function called ceph_mdsc_build_path(), which builds a string representation of a file’s full path.

The Technical Root

The problem starts if Ceph is handed a deeply nested file or directory whose path length exceeds PATH_MAX (the maximum path length the kernel allows, typically 4096 bytes). Instead of giving up and returning an error like ENAMETOOLONG, the function would try again... and again... and again... forever.

This looks roughly like

// Pseudo-code representation, simplified for explanation
int ceph_mdsc_build_path(struct inode *inode, char *buf, int buflen) {
    int try = ;
    // ...some logic...
    while (need_to_retry) {
        if (path_too_long)
            continue; // Keeps trying forever!
        try++;
        if (try > 100)
            break;
    }
    // Never actually breaks for very long paths!
}

As a result, the function traps the kernel thread in an endless retry loop, effectively hogging CPU and causing the host to become unresponsive or slow to a crawl. This is why CVE-2024-53685 is an easy (and embarrassing) *Denial-of-Service*—any user with the right permissions could bring down a machine by simply creating a super long Ceph file path.

The Real Patch: Short and Sweet

The fix was smart: just stop retrying and *fail gracefully*. When the path is over PATH_MAX, immediately return an error (ENAMETOOLONG). Here’s the link to the patch, and the crucial fix itself:

// Before: endless retry
if (path_too_long)
    continue; // Blocks forever

// After: fail immediately
if (path_too_long)
    return -ENAMETOOLONG;

Just two lines of code prevent the worst. No retries, no infinite loops, no simple DoS.

Suppose you’re on a system with Ceph and have enough permissions. You run

mkdir /mnt/ceph/$(python3 -c 'print("a/" * 500)')

This makes a nested directory path that totals well over 4096 bytes.

With the old kernel:

Kernel task gets stuck and the machine goes unresponsive.

With the fixed kernel:

Any recent Linux system using Ceph kernel client before this patch.

- Impact: Anyone able to create files/directories (even unprivileged users in some setups) could trigger the bug and lock up a server.

Even if you don’t use Ceph yourself, big storage clusters or companies with multiuser Linux environments might be impacted.

Monitor Unusual File Paths:

Watch for unusually long file/directory creation attempts as a possible attack sign.

Limit untrusted users' ability to create entries on sensitive Ceph mounts.

## Learn More / Official References

- Kernel Patch Discussion
- CVE Record
- Upstream Linux commit
- Ceph Documentation

Final Thoughts

CVE-2024-53685 is a great reminder that "try again" logic, if not carefully capped, can break things badly. Always handle errors (especially ENAMETOOLONG!) and don’t assume retries are harmless.

Keep your Linux kernel up to date, especially if you rely on complex filesystems like Ceph. Sometimes, the biggest breakdowns start with a single line of code—and end with a simple fix!

*Written exclusively for you, with clear breakdowns and original examples. Stay safe!*

Timeline

Published on: 01/11/2025 13:15:25 UTC
Last modified on: 05/04/2025 09:56:54 UTC