---
id: CVE-2025-39977
title: 'futex: Prevent use-after-free during requeue-PI'
summary: |-
  In the Linux kernel, the following vulnerability has been resolved:

  futex: Prevent use-after-free during requeue-PI

  syzbot managed to trigger the following race:

     T1                               T2

   futex_wait_requeue_pi()
     fute…
severity: high
cvss: 7.8
cvssVector: 'CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H'
cvssSource: cna
vendor: Linux
product: Linux
affected:
  - >-
    Linux >= 07d91ef510fb16a2e0ca7453222105835b7ba3b8 <
    cb5d19a61274b51b49601214a87af573b43d60fa
  - >-
    Linux >= 07d91ef510fb16a2e0ca7453222105835b7ba3b8 <
    348736955ed6ca6e99ca24b93b1d3fbfe352c181
  - >-
    Linux >= 07d91ef510fb16a2e0ca7453222105835b7ba3b8 <
    a170b9c0dde83312b8b58ccc91509c7c15711641
  - >-
    Linux >= 07d91ef510fb16a2e0ca7453222105835b7ba3b8 <
    d824b2dbdcfe3c390278dd9652ea526168ef6850
  - >-
    Linux >= 07d91ef510fb16a2e0ca7453222105835b7ba3b8 <
    b549113738e8c751b613118032a724b772aa83f2
  - Linux 5.15
published: '2025-10-15'
updated: '2026-09-08'
sourceUpdated: '2026-09-08T08:42:39.132Z'
source: CVEORG
sourceUrl: 'https://www.cve.org/CVERecord?id=CVE-2025-39977'
references:
  - url: 'https://git.kernel.org/stable/c/cb5d19a61274b51b49601214a87af573b43d60fa'
  - url: 'https://git.kernel.org/stable/c/348736955ed6ca6e99ca24b93b1d3fbfe352c181'
  - url: 'https://git.kernel.org/stable/c/a170b9c0dde83312b8b58ccc91509c7c15711641'
  - url: 'https://git.kernel.org/stable/c/d824b2dbdcfe3c390278dd9652ea526168ef6850'
  - url: 'https://git.kernel.org/stable/c/b549113738e8c751b613118032a724b772aa83f2'
tags:
  - cve.org
epss: 0.0015
epssPercentile: 0.035
ingestedAt: '2026-09-08T15:33:26.996Z'
---

## Overview

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

futex: Prevent use-after-free during requeue-PI

syzbot managed to trigger the following race:

   T1                               T2

 futex_wait_requeue_pi()
   futex_do_wait()
     schedule()
                               futex_requeue()
                                 futex_proxy_trylock_atomic()
                                   futex_requeue_pi_prepare()
                                   requeue_pi_wake_futex()
                                     futex_requeue_pi_complete()
                                      /* preempt */

         * timeout/ signal wakes T1 *

   futex_requeue_pi_wakeup_sync() // Q_REQUEUE_PI_LOCKED
   futex_hash_put()
  // back to userland, on stack futex_q is garbage

                                      /* back */
                                     wake_up_state(q->task, TASK_NORMAL);

In this scenario futex_wait_requeue_pi() is able to leave without using
futex_q::lock_ptr for synchronization.

This can be prevented by reading futex_q::task before updating the
futex_q::requeue_state. A reference on the task_struct is not needed
because requeue_pi_wake_futex() is invoked with a spinlock_t held which
implies a RCU read section.

Even if T1 terminates immediately after, the task_struct will remain valid
during T2's wake_up_state().  A READ_ONCE on futex_q::task before
futex_requeue_pi_complete() is enough because it ensures that the variable
is read before the state is updated.

Read futex_q::task before updating the requeue state, use it for the
following wakeup.

## Affected

- `Linux >= 07d91ef510fb16a2e0ca7453222105835b7ba3b8 < cb5d19a61274b51b49601214a87af573b43d60fa`
- `Linux >= 07d91ef510fb16a2e0ca7453222105835b7ba3b8 < 348736955ed6ca6e99ca24b93b1d3fbfe352c181`
- `Linux >= 07d91ef510fb16a2e0ca7453222105835b7ba3b8 < a170b9c0dde83312b8b58ccc91509c7c15711641`
- `Linux >= 07d91ef510fb16a2e0ca7453222105835b7ba3b8 < d824b2dbdcfe3c390278dd9652ea526168ef6850`
- `Linux >= 07d91ef510fb16a2e0ca7453222105835b7ba3b8 < b549113738e8c751b613118032a724b772aa83f2`
- `Linux 5.15`

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.