In June 2024, the Linux kernel team fixed a tricky bug that caused issues when using io_uring, uncovered by automated testing. This bug, now tracked as CVE-2024-56584, can lead to kernel panic via a failed assertion check caused by memory allocation issues in the xarray data structure. Here's a detailed, plain-language breakdown of the issue, the vulnerable code, its implications, and how it was fixed.

What was Affected?

- Component: io_uring (high-performance async IO), specifically io_uring/tctx.c.

Kernel Version: Primarily seen in 6.8.x and 6.9 release lines.

- Issue Found By: syzbot, a kernel fuzzer.

Background

io_uring allows user applications to submit IO requests without entering the kernel for every operation, improving performance. It relies on efficient in-kernel data management—here, using xarrays to map requests. The bug involves how xarray (xa_store()) handles out-of-memory situations.

Symptom: Kernel Warning

When memory allocation fails during an xa_store() operation, the xarray may end up in a state where its ->head pointer is not NULL, yet it contains zero valid entries. Later, kernel code running in __io_uring_free() tries to assert the array is empty with WARN_ON_ONCE(!xa_empty(&tctx->xa));. This triggers a WARN, and possibly a kernel panic or instability.

Error Log Example

WARNING: CPU:  PID: 16 at io_uring/tctx.c:51 __io_uring_free+xfa/x140 io_uring/tctx.c:51

Problem Code (Before Fix)

void __io_uring_free(struct io_uring_task *tctx)
{
    WARN_ON_ONCE(!xa_empty(&tctx->xa));
    // cleanup...
}

If xa_store() fails on memory allocation, xa_empty() may incorrectly return false because of stray ->head.

How Was It Found?

syzbot, a kernel fuzzing tool, injects memory allocation failures. Its tests triggered the bug, showing that the kernel did not properly handle all resource cleanup paths when allocations fail inside xa_store().

Original report

- syzbot crash report

Exploit Scenario

While this bug does not directly enable a user to escalate privileges, denial-of-service is possible. By repeatedly triggering IO operations while under forced memory pressure (e.g., with a local unprivileged process), an attacker could cause the system to hit the warning and either panic the kernel or leave it in a bad state.

Sample Exploit Logic

// Pseudocode for local DoS
for (;;) {
    inject_mem_allocation_failure();
    submit_io_uring_request();
    // System may trip the WARN_ON in __io_uring_free
}

Tools like syzbot automate these steps, but you could write your own harness using the Linux kernel's fault injection interfaces and the io_uring API.

The Quick Fix

Until the underlying xarray behavior is redesigned, the kernel now works around this by checking for stray entries explicitly rather than assuming xa_empty()'s correctness. It will iterate through xarray's entries, warning if any are found.

Patch Excerpt (After Fix)

See the patch discussion:

unsigned long index = ;
void *entry;

xa_for_each(&tctx->xa, index, entry) {
    WARN_ON_ONCE(entry != NULL);
}

So if a rogue entry exists, it gets flagged explicitly.

- Linux Patch Lore Archive
- syzbot bug entry
- io_uring upstream kernel source

Key Takeaways

- CVE-2024-56584 can cause Linux systems using io_uring to panic under low-memory stress via local user interaction.
- It exposes how subtle memory handling bugs in performance-critical kernel paths can still bite, especially under conditions only fuzzers will hit.
- The bug is not a privilege escalation, but is a local denial-of-service risk. Update your kernel or apply the patch if you rely on io_uring.

How to Stay Safe

- Upgrade your kernel to a version with this fix (check via your vendor's security advisories or kernel mailing list).

If you run Linux with io_uring support—especially on multiuser systems—patch promptly!

*Content original to this post. For questions, contact your Linux vendor or check the references above.*

Timeline

Published on: 12/27/2024 15:15:17 UTC
Last modified on: 05/04/2025 09:59:02 UTC