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Fencer Vulns
medium 5.5

CVE-2026-31555 json

futex: Clear stale exiting pointer in futex_lock_pi() retry path

published 2026-04-24 6 mo ago modified 2026-11-05 3 d ago also EUVD-2026-25448

cvss
5.5
version 3.1, NVD
cisa kev
Not listed
not in CISA's catalog
epss
0.1%
EPSS 0.1% chance of exploitation in 30 days, percentile 2%
public exploits
0
none known

Summary

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

futex: Clear stale exiting pointer in futex_lock_pi() retry path

Fuzzying/stressing futexes triggered:

WARNING: kernel/futex/core.c:825 at wait_for_owner_exiting+0x7a/0x80, CPU#11: futex_lock_pi_s/524

When futex_lock_pi_atomic() sees the owner is exiting, it returns -EBUSY and stores a refcounted task pointer in 'exiting'.

After wait_for_owner_exiting() consumes that reference, the local pointer is never reset to nil. Upon a retry, if futex_lock_pi_atomic() returns a different error, the bogus pointer is passed to wait_for_owner_exiting().

CPU0 CPU1 CPU2 futex_lock_pi(uaddr) // acquires the PI futex exit() futex_cleanup_begin() futex_state = EXITING; futex_lock_pi(uaddr) futex_lock_pi_atomic() attach_to_pi_owner() // observes EXITING *exiting = owner; // takes ref return -EBUSY wait_for_owner_exiting(-EBUSY, owner) put_task_struct(); // drops ref // exiting still points to owner goto retry; futex_lock_pi_atomic() lock_pi_update_atomic() cmpxchg(uaddr) *uaddr ^= WAITERS // whatever // value changed return -EAGAIN; wait_for_owner_exiting(-EAGAIN, exiting) // stale WARN_ON_ONCE(exiting)

Fix this by resetting upon retry, essentially aligning it with requeue_pi.

Patch / fix

  • Linux Linux 4.5, 4.10, 4.15, 4.20, 5.5, 5.10.253, 5.15.203, 6.1.168, 6.6.131, 6.12.80, 6.18.21, 6.19.11, 7.0
  • linux Debian 12 6.1.170-1
  • linux Debian 13 6.12.85-1
  • linux Debian 14 6.19.11-1

debian advisories DEBIAN-CVE-2026-31555

  • linux Ubuntu 22.04 LTS 5.15.0-186.196
  • linux Ubuntu 24.04 LTS 6.8.0-136.136
  • linux-aws Ubuntu 22.04 LTS 5.15.0-1112.119
  • linux-aws Ubuntu 24.04 LTS 6.8.0-1061.64
  • linux-gcp Ubuntu 22.04 LTS 5.15.0-1112.122
  • linux-gcp Ubuntu 24.04 LTS 6.8.0-1064.72
  • linux-gke Ubuntu 22.04 LTS 5.15.0-1108.114
  • linux-gke Ubuntu 24.04 LTS 6.8.0-1059.67

ubuntu advisories UBUNTU-CVE-2026-31555

Ubuntu security data © Canonical Ltd., from https://github.com/canonical/ubuntu-security-notices via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

  • kernel RHEL 9 5.14.0-687.36.1.el9_8
  • rv RHEL 9 5.14.0-687.36.1.el9_8
  • perf RHEL 9 5.14.0-687.36.1.el9_8
  • rtla RHEL 9 5.14.0-687.36.1.el9_8
  • libperf RHEL 9 5.14.0-687.36.1.el9_8
  • kernel-rt RHEL 9 5.14.0-687.36.1.el9_8
  • kernel-64k RHEL 9 5.14.0-687.36.1.el9_8
  • kernel-core RHEL 9 5.14.0-687.36.1.el9_8

red hat advisories RHSA-2026:51035

  • kernel-syms SLES 16.0, Linux Micro Extras 6.2, SLES for SAP applications 16.0 6.12.0-160000.36.1
  • kernel-obs-build Linux Micro Extras 6.2 6.12.0-160000.36.1
  • kernel-rt Linux Micro 6.2 6.12.0-160000.36.1
  • kernel-64kb SLES 16.0, Linux Micro 6.2, SLES for SAP applications 16.0 6.12.0-160000.36.1
  • kernel-source SLES 16.0, Linux Micro 6.2, SLES for SAP applications 16.0 6.12.0-160000.36.1
  • kernel-default SLES 16.0, SLE High Availability Extension 16.0, Linux Micro 6.2 +1 more 6.12.0-160000.36.1
  • kernel-default-base SLES 16.0, Linux Micro 6.2, SLES for SAP applications 16.0 6.12.0-160000.36.1.160000.2.17
  • kernel-azure SLES for SAP applications 16.0, SLES 16.0 6.12.0-160000.36.1

suse advisories SUSE-SU-2026:22742-1SUSE-SU-2026:22769-1SUSE-SU-2026:22835-1SUSE-SU-2026:22812-1SUSE-SU-2026:2799-1SUSE-SU-2026:2722-1

  • kernel-rt openSUSE Leap 16.0 6.12.0-160000.36.1
  • dtb-aarch64 openSUSE Leap 16.0 6.12.0-160000.36.1
  • kernel-64kb openSUSE Leap 16.0 6.12.0-160000.36.1
  • kernel-syms openSUSE Leap 16.0 6.12.0-160000.36.1
  • kernel-azure openSUSE Leap 16.0 6.12.0-160000.36.1
  • kernel-obs-qa openSUSE Leap 16.0 6.12.0-160000.36.1
  • kernel-source openSUSE Leap 16.0 6.12.0-160000.36.1
  • kernel-default openSUSE Leap 16.0 6.12.0-160000.36.1

opensuse advisories openSUSE-SU-2026:21388-1

  • kernel AlmaLinux 9 5.14.0-687.36.1.el9_8
  • rv AlmaLinux 9 5.14.0-687.36.1.el9_8
  • perf AlmaLinux 9 5.14.0-687.36.1.el9_8
  • rtla AlmaLinux 9 5.14.0-687.36.1.el9_8
  • libperf AlmaLinux 9 5.14.0-687.36.1.el9_8
  • kernel-rt AlmaLinux 9 5.14.0-687.36.1.el9_8
  • kernel-64k AlmaLinux 9 5.14.0-687.36.1.el9_8
  • kernel-core AlmaLinux 9 5.14.0-687.36.1.el9_8

almalinux advisories ALSA-2026:51035

  • kernel Azure Linux 3.0 6.6.134.1-2

azure linux advisories AZL-83633

Azure Linux vulnerability data, Copyright (c) Microsoft Corporation, from https://github.com/microsoft/AzureLinuxVulnerabilityData via OSV, under the MIT licence; changed: normalised to fixed package versions per release. MIT

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2026-31555 updated cvss 5.5

kernel: futex: Clear stale exiting pointer in futex_lock_pi() retry path

  • Red Hat Enterprise Linux 7 perf, kernel, bpftool, kernel-rt, python-perf, kernel-kdump, +8 more Red Hat Enterprise Linux 8 perf, kernel, bpftool, kernel-rt, kernel-core, kernel-tools, +29 more Red Hat Enterprise Linux 9 kernel-rt, kernel-rt-kvm, kernel-rt-core, kernel-rt-modules, kernel-rt-debug-kvm, kernel-rt-debug-core, +12 more Red Hat Enterprise Linux 10 kernel
    affected: no fix available “Fix deferred”
  • Red Hat Enterprise Linux AppStream (v. 9) rv 5.14.0-687.36.1.el9_8, perf 5.14.0-687.36.1.el9_8, rtla 5.14.0-687.36.1.el9_8, python3-perf 5.14.0-687.36.1.el9_8, kernel-devel-matched 5.14.0-687.36.1.el9_8, kernel-64k-devel-matched 5.14.0-687.36.1.el9_8, +7 more Red Hat Enterprise Linux BaseOS (v. 9) kernel 5.14.0-687.36.1.el9_8, kernel-64k 5.14.0-687.36.1.el9_8, kernel-core 5.14.0-687.36.1.el9_8, kernel-tools 5.14.0-687.36.1.el9_8, kernel-modules 5.14.0-687.36.1.el9_8, kernel-64k-core 5.14.0-687.36.1.el9_8, +27 more Red Hat Enterprise Linux CodeReady Linux Builder (v. 9) libperf 5.14.0-687.36.1.el9_8, kernel-debuginfo-common-s390x 5.14.0-687.36.1.el9_8, kernel-debuginfo-common-x86_64 5.14.0-687.36.1.el9_8, kernel-debuginfo-common-aarch64 5.14.0-687.36.1.el9_8, kernel-debuginfo-common-ppc64le 5.14.0-687.36.1.el9_8 Red Hat Enterprise Linux Real Time (v. 9) kernel-rt 5.14.0-687.36.1.el9_8, kernel-rt-64k 5.14.0-687.36.1.el9_8, kernel-rt-core 5.14.0-687.36.1.el9_8, kernel-rt-modules 5.14.0-687.36.1.el9_8, kernel-rt-64k-core 5.14.0-687.36.1.el9_8, kernel-rt-debug-core 5.14.0-687.36.1.el9_8, +14 more Red Hat Enterprise Linux Real Time for NFV (v. 9) kernel-rt 5.14.0-687.36.1.el9_8, kernel-rt-core 5.14.0-687.36.1.el9_8, kernel-rt-modules 5.14.0-687.36.1.el9_8, kernel-rt-debug-core 5.14.0-687.36.1.el9_8, kernel-rt-modules-core 5.14.0-687.36.1.el9_8, kernel-rt-debug-modules 5.14.0-687.36.1.el9_8, +4 more
    fixed: vendor fix RHSA-2026:51035
  • Red Hat Enterprise Linux 6 perf, kernel, python-perf, kernel-kdump, kernel-firmware, kernel-bootwrapper, +1 more
    not affected: vulnerable code not present

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

cisa ICSA-26-295-11 updated cvss 5.5

Siemens SIMATIC S7-1500 CPU 1518(F)-4 PN/DP MFP

  • Siemens SIMATIC S7-1500 CPU 1518-4 PN/DP MFP (6ES7518-4AX00-1AB0) >= 3.1.7 Siemens SIMATIC S7-1500 CPU 1518-4 PN/DP MFP (6ES7518-4AX00-1AC0) >= 3.1.7 Siemens SIMATIC S7-1500 CPU 1518F-4 PN/DP MFP (6ES7518-4FX00-1AB0) >= 3.1.7 Siemens SIMATIC S7-1500 CPU 1518F-4 PN/DP MFP (6ES7518-4FX00-1AC0) >= 3.1.7 Siemens SIPLUS S7-1500 CPU 1518-4 PN/DP MFP (6AG1518-4AX00-4AC0) >= 3.1.7
    affected: mitigation, no fix available

Facts from CISA's republication of the vendor's own advisory (CSAF); the linked advisory is the authoritative source. CISA notification · csaf

cisa ICSA-26-209-04 updated cvss 5.5

Siemens SIMATIC S7-1500 CPU 1518(F)-4 PN/DP MFP

  • Siemens SIMATIC S7-1500 CPU 1518-4 PN/DP MFP (6ES7518-4AX00-1AB0) >= 3.1.6 Siemens SIMATIC S7-1500 CPU 1518-4 PN/DP MFP (6ES7518-4AX00-1AC0) >= 3.1.6 Siemens SIMATIC S7-1500 CPU 1518F-4 PN/DP MFP (6ES7518-4FX00-1AB0) >= 3.1.6 Siemens SIMATIC S7-1500 CPU 1518F-4 PN/DP MFP (6ES7518-4FX00-1AC0) >= 3.1.6 Siemens SIPLUS S7-1500 CPU 1518-4 PN/DP MFP (6AG1518-4AX00-4AC0) >= 3.1.6
    affected: mitigation, no fix available

Facts from CISA's republication of the vendor's own advisory (CSAF); the linked advisory is the authoritative source. CISA notification · csaf

CISA SSVC

Exploitation
none
Automatable
no
Technical impact
partial
Assessed
3 d ago

Exploit prediction

0.13% chance of exploitation in the next 30 days, higher than 2% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
NVD 3.1 5.5 CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

Affected products

  • Linux Linux
  • Siemens SIMATIC S7-1500 CPU 1518-4 PN/DP MFP
  • Siemens SIMATIC S7-1500 CPU 1518F-4 PN/DP MFP
  • Siemens SIPLUS S7-1500 CPU 1518-4 PN/DP MFP

Classification

Technology feeds

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.