CVE-2024-39508 - Data Race in Linux Kernel io_uring/io-wq Fixed with Atomic Bit Operations

On modern Linux systems, the io_uring interface brings high-performance asynchronous I/O, powering everything from database backends to web servers. However, with speed comes complexity—and occasionally, that complexity means bugs creep in.

CVE-2024-39508 is a recently fixed data race vulnerability in Linux’s io_uring subsystem, particularly involving the io-wq (workqueue) code. Let’s break down what happened, how it was fixed, show code examples, and offer links for further reading.

TL;DR

A data race existed due to non-atomic access to the flags field in the io_worker structure. This could result in undefined or buggy behavior on multiprocessor systems, especially under heavy load.

How Was the Bug Found?

Developers running Linux with KCSAN (Kernel Concurrency Sanitizer) enabled began to get noisy bug reports. KCSAN instruments reads and writes to shared variables, catching concurrency issues:

BUG: KCSAN: data-race in io_worker_handle_work / io_wq_activate_free_worker
write to xffff8885c4246404 of 4 bytes by task 49071 on cpu 28:
  io_worker_handle_work (io_uring/io-wq.c:434 io_uring/io-wq.c:569)
...
read to xffff8885c4246404 of 4 bytes by task 49024 on cpu 5:
  io_wq_activate_free_worker (io_uring/io-wq.c:? io_uring/io-wq.c:285)

What this means: Two kernels threads, running on two CPUs, accessed the *same* worker flag variable at *almost* the same time—one writing, the other reading, with no coordination.

Why Is This Dangerous?

When multiple CPUs touch the same memory without synchronization (atomic operations or locks), you can get:

Bit flips or half-written values

- Workers not waking/sleeping as expected

The worker’s flags were manipulated directly via bitwise operations

// Potentially racy old code (DO NOT USE)
worker->flags |= IO_WORKER_F_FREE;
if (worker->flags & IO_WORKER_F_FREE) {
    // ...
}

With several threads doing this, atomicity is NOT guaranteed!

After the patch addressing CVE-2024-39508, atomic bit operations are used

// Thread-safe manipulation after the patch
set_bit(IO_WORKER_F_FREE, &worker->flags);

if (test_bit(IO_WORKER_F_FREE, &worker->flags)) {
    // Correctly checks the flag, safely
}

clear_bit(IO_WORKER_F_BUSY, &worker->flags);  // Example of clearing atomically

These routines—set_bit, test_bit, clear_bit—are *atomic*, meaning the kernel ensures they run to completion without interruption.

Could It Be Exploited?

This particular race condition is tricky to exploit directly—it’s not a local privilege escalation or buffer overflow. Instead, the danger is:

Potential for kernel panics, resource leaks, or processes getting stuck

Attackers could theoretically trigger high loads and strand workqueue workers, but direct exploitation is unlikely.

## Official Links / References

- Kernel Patch Commit
- KCSAN Documentation
- io_uring Design Documentation
- io_uring Upstream Code

Other Fixes: Structure Padding

In addition to fixing the data race, the patch moved the create_index field inside the structure to prevent unwanted holes (gaps in memory layout).

Why? Fields spread with unused space ("holes") can make races and memory corruption bugs even harder to track and fix. Reordering the structure reduces hidden side effects.

Conclusion

If you run modern Linux with io_uring workloads, update to a kernel that contains this fix! While direct exploits are unlikely, any data races in the kernel can cause hard-to-diagnose issues or, eventually, security risks.

In summary:
Always use atomic operations to coordinate shared data between CPUs or threads, especially in the Linux kernel!

Timeline

Published on: 07/12/2024 13:15:13 UTC
Last modified on: 05/04/2025 09:17:18 UTC