# CVE-2026-99252: io_uring/mpscq: disable preemption while pushing

Status: published. Published: 2026-11-03. Updated: 2026-11-03.

In the Linux kernel, the following vulnerability has been resolved:

io_uring/mpscq: disable preemption while pushing

A push makes the node the new tail, and then links it to its predecessor.
In between the queue isn't empty, but mpscq_pop() can't get at the node.
The consumers deal with that by trying again, with a cond_resched() in
the loop to give a preempted producer the chance to finish.

cond_resched() doesn't do that if the consumer has a higher priority than
the producer and both have to use the same CPU:

  producer (SCHED_OTHER)              consumer (SCHED_FIFO), same CPU
  io_req_local_work_add()
    mpscq_push()
      prev = xchg(&q->tail, node)
  <preempted>                         io_cqring_wait()
                                        __io_run_local_work()
                                      again:
                                          cond_resched()  // nothing to do
                                          mpscq_pop()     // NULL
                                          // work pending, no events yet
                                          goto again

The producer can be any task that is able to queue task_work for the
ring: an io-wq worker, or somebody who sends a message with
IORING_OP_MSG_RING. The consumer spins, holding ->uring_lock, until the
scheduler takes pity on the fair class, which takes about a second with
the default settings of the fair server, and forever without it. The
loops in tctx_task_work_run() and io_cancel_local_task_work() have the
same problem.

Don't allow preemption between the two steps. That bounds the time the
window is open on the CPU of the producer by what interrupts take, and a
consumer that waits for that is on another CPU. The cost is a
preempt_disable()/preempt_enable() pair per push.

## Remediation

No remediation reported in collected sources

## Affected products and versions

- Linux / Linux: ≥ 50cb44bd0d5f243919a06b17b1d979fdcd72cb2b; < 8c94677e2b6145393d4c129fe0f5364b0e5000a5 (affected, git). Source: CNA: Linux
- Linux / Linux: 7.2 (affected); ≥ 0; < 7.2 (unaffected, semver); ≥ 7.2.14; ≤ 7.2.* (unaffected, semver). Source: CNA: Linux

## Severity and prioritization

- CISA KEV: No listing in collected sources

- EPSS: Not available

- SSVC: Not available

- Weaknesses: Not reported

## Exploit evidence

No exploit evidence in collected sources. This is not proof of no exploitation.

## References

- https://git.kernel.org/stable/c/8c94677e2b6145393d4c129fe0f5364b0e5000a5 (CNA: Linux)

## Provenance

- CNA: Linux: https://www.cve.org/CVERecord?id=CVE-2026-99252; retrieved 2026-11-03; source updated 2026-11-03

Revision: 0000c2f9aa0e4bf98c5b1b2c2e78f07ef0e8dae9bf7e7b92dbad52a6da815a59

Canonical: https://y2.dev/cve/CVE-2026-99252

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