In late 2023, the Linux kernel team resolved a subtle but significant vulnerability, recorded as CVE-2023-52656. The issue stemmed from legacy code in io_uring, one of Linux’s most modern asynchronous I/O interfaces. The crux? Residual code supporting SCM_RIGHTS—a mechanism for passing file descriptors over Unix sockets even though io_uring had already dropped such support.

This post will break down how the vulnerability existed, why it was risky, how the fix worked, and even provide code comparisons. We’ll keep everything straightforward and beginner-friendly, using real code and sources so curious minds can dig deeper.

What is io_uring and SCM_RIGHTS?

- io_uring: A high-speed asynchronous I/O interface for Linux (since 5.1), enabling non-blocking read/write operations at blazing speeds.
- SCM_RIGHTS: A special Unix “control message” (ancient but powerful!) that allows passing file descriptors between processes using sockets. Think of it as a secret pipe for passing access to files, without opening them again.

The Vulnerability: Ghosts in the Code

When io_uring first launched, there was support for passing its own file descriptors through SCM_RIGHTS. As io_uring evolved, this feature was dropped—but a chunk of code related to SCM_RIGHTS still lingered around in the kernel source.

Residual or “dead” code can open up unexpected security holes, especially around complex kernel functions. Leftover SCM_RIGHTS handling meant attackers could potentially interact with legacy paths, perhaps triggering edge cases, or worse—crashes, data leakage, or undefined behavior.

Before (Vulnerable / Dead Code Present)

// net/unix/af_unix.c (outdated snippet, for illustration)
#ifdef CONFIG_IO_URING
    if (cmsg->cmsg_level == SOL_SOCKET &&
        cmsg->cmsg_type == SCM_RIGHTS) {
        // Legacy io_uring fd support
        // This code was dead but still present!
        ...
    }
#endif

After (Fixed/Patched)

// All references to io_uring + SCM_RIGHTS case have been scrubbed
// File now looks like:
if (cmsg->cmsg_level == SOL_SOCKET &&
    cmsg->cmsg_type == SCM_RIGHTS) {
    // Regular fd passing code, nothing about io_uring anymore.
    ...
}

While the dead code wasn’t actively exposing io_uring file descriptors anymore, it represented

- Increased attack surface: Attackers love dormant code; it gets less attention and fewer bug fixes.

Inconsistencies: Dead code can interact in weird ways with new kernel logic.

- Future regressions: Developers could mistakenly resurrect or misuse dead logic, introducing vulnerabilities.

How Was CVE-2023-52656 Fixed?

Pretty elegantly: All SCM_RIGHTS-related code that referenced io_uring was deleted.

See the official patch by Jens Axboe:

> > "io_uring: drop any code related to SCM_RIGHTS <br>> > This is dead code after we dropped support for passing io_uring fds over SCM_RIGHTS, get rid of it."
> > — Jens Axboe, kernel developer.

Exploitability: What Could a Bad Actor Do?

Direct active exploits aren’t widely circulating (since code was already functionally “dead”), but a determined attacker could have:

Probed unused io_uring code paths.

- Triggered special crafted socket messages, attempting to exploit edge behavior (kernel panics, unexpected reference leaks).

Prepared for future exploits if anyone tried to reuse the code without realizing the baggage.

Proof of Concept:
Let’s imagine an attacker poked at the SCM_RIGHTS handler, sending sketchy cmsgs and bogus file descriptors. While prevented from directly taking over, such attacks could reveal subtle bugs. With dead code, security researchers often find use-after-free, double-close, or memory-leak bugs here.

How to Stay Safe

- Upgrade your kernel to at least 6.8, or any release containing the fix.

References & Further Reading

- Kernel commit dropping SCM_RIGHTS support in io_uring
- Linux Weekly News Patch Summary
- CVE report for CVE-2023-52656
- io_uring documentation (Kernel docs)
- Jens Axboe’s glm: email patch discussion

Conclusion

CVE-2023-52656 is a great modern example of how legacy code in even the fastest new Linux tech can become a lurking danger. The fix is simple—delete what is no longer used—but the lesson is timeless: stay vigilant, clean up, and always remember that even “dead” code can bite.

If you’re running production Linux with io_uring, check your kernel, apply updates, and sleep better at night!

Timeline

Published on: 05/14/2024 14:23:13 UTC
Last modified on: 05/04/2025 07:40:58 UTC