# CVE-2026-31555: futex: Clear stale exiting pointer in futex_lock_pi() retry path

Status: published. Published: 2026-04-24. Updated: 2026-11-05.

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.

## Remediation

No remediation reported in collected sources

## Affected products and versions

- Linux / Linux: ≥ 3ef240eaff36b8119ac9e2ea17cbf41179c930ba; < 33095ae3bdde5e5c264d7e88a2f3e7703a26c7aa (affected, git); ≥ 3ef240eaff36b8119ac9e2ea17cbf41179c930ba; < e7824ec168d2ac883a213cd1f4d6cc0816002a85 (affected, git); ≥ 3ef240eaff36b8119ac9e2ea17cbf41179c930ba; < 5e8e06bf8909e79b4acd950cf578cfc2f10bbefa (affected, git); ≥ 3ef240eaff36b8119ac9e2ea17cbf41179c930ba; < de7c0c04ad868f2cee6671b11c0a6d20421af1da (affected, git); ≥ 3ef240eaff36b8119ac9e2ea17cbf41179c930ba; < 7475dfad10a05a5bfadebf5f2499bd61b19ed293 (affected, git); ≥ 3ef240eaff36b8119ac9e2ea17cbf41179c930ba; < 92e47ad03e03dbb5515bdf06444bf6b1e147310d (affected, git); ≥ 3ef240eaff36b8119ac9e2ea17cbf41179c930ba; < 71112e62807d1925dc3ae6188b11f8cfc85aec23 (affected, git); ≥ 3ef240eaff36b8119ac9e2ea17cbf41179c930ba; < 210d36d892de5195e6766c45519dfb1e65f3eb83 (affected, git); f2a9957e5c08b1b1caacd18a3dc4c0a1bdb7b463 (affected, git); cf16e42709aa86aa3e37f3acc3d13d5715d90096 (affected, git); 61fa9f167caaa73d0a7c88f498eceeb12c6fa3db (affected, git); 7874eee0130adf9bee28e8720bb5dd051089def3 (affected, git); fc3b55ef2c840bb2746b2d8121a0788de84f7fac (affected, git); ≥ 4.4.255; < 4.5 (affected, semver); ≥ 4.9.255; < 4.10 (affected, semver); ≥ 4.14.158; < 4.15 (affected, semver); ≥ 4.19.172; < 4.20 (affected, semver); ≥ 5.4.1; < 5.5 (affected, semver). Source: CNA: Linux
- Linux / Linux: 5.5 (affected); ≥ 0; < 5.5 (unaffected, semver); ≥ 5.10.253; ≤ 5.10.* (unaffected, semver); ≥ 5.15.203; ≤ 5.15.* (unaffected, semver); ≥ 6.1.168; ≤ 6.1.* (unaffected, semver); ≥ 6.6.131; ≤ 6.6.* (unaffected, semver); ≥ 6.12.80; ≤ 6.12.* (unaffected, semver); ≥ 6.18.21; ≤ 6.18.* (unaffected, semver); ≥ 6.19.11; ≤ 6.19.* (unaffected, semver); ≥ 7.0; ≤ * (unaffected, original_commit_for_fix). Source: CNA: Linux
- Siemens / SIMATIC S7-1500 CPU 1518-4 PN/DP MFP: ≥ V3.1.6; < * (affected, custom). Source: ADP: siemens-SADP
- Siemens / SIMATIC S7-1500 CPU 1518-4 PN/DP MFP: ≥ V3.1.5; < * (affected, custom). Source: ADP: siemens-SADP
- Siemens / SIMATIC S7-1500 CPU 1518-4 PN/DP MFP: ≥ V3.1.7; < * (affected, custom). Source: ADP: siemens-SADP
- Siemens / SIMATIC S7-1500 CPU 1518F-4 PN/DP MFP: ≥ V3.1.6; < * (affected, custom). Source: ADP: siemens-SADP
- Siemens / SIMATIC S7-1500 CPU 1518F-4 PN/DP MFP: ≥ V3.1.5; < * (affected, custom). Source: ADP: siemens-SADP
- Siemens / SIMATIC S7-1500 CPU 1518F-4 PN/DP MFP: ≥ V3.1.7; < * (affected, custom). Source: ADP: siemens-SADP
- Siemens / SIPLUS S7-1500 CPU 1518-4 PN/DP MFP: ≥ V3.1.6; < * (affected, custom). Source: ADP: siemens-SADP
- Siemens / SIPLUS S7-1500 CPU 1518-4 PN/DP MFP: ≥ V3.1.5; < * (affected, custom). Source: ADP: siemens-SADP
- Siemens / SIPLUS S7-1500 CPU 1518-4 PN/DP MFP: ≥ V3.1.7; < * (affected, custom). Source: ADP: siemens-SADP

## Severity and prioritization

- CISA KEV: No listing in collected sources

- EPSS: 0.13% as of 2026-11-05; percentile 2.53%

- SSVC: exploitation none; automatable no; technical impact partial; source ADP: CISA-ADP

- 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/33095ae3bdde5e5c264d7e88a2f3e7703a26c7aa (CNA: Linux)

- https://git.kernel.org/stable/c/e7824ec168d2ac883a213cd1f4d6cc0816002a85 (CNA: Linux)

- https://git.kernel.org/stable/c/5e8e06bf8909e79b4acd950cf578cfc2f10bbefa (CNA: Linux)

- https://git.kernel.org/stable/c/de7c0c04ad868f2cee6671b11c0a6d20421af1da (CNA: Linux)

- https://git.kernel.org/stable/c/7475dfad10a05a5bfadebf5f2499bd61b19ed293 (CNA: Linux)

- https://git.kernel.org/stable/c/92e47ad03e03dbb5515bdf06444bf6b1e147310d (CNA: Linux)

- https://git.kernel.org/stable/c/71112e62807d1925dc3ae6188b11f8cfc85aec23 (CNA: Linux)

- https://git.kernel.org/stable/c/210d36d892de5195e6766c45519dfb1e65f3eb83 (CNA: Linux)

- https://cert-portal.siemens.com/productcert/html/ssa-082556.html (ADP: siemens-SADP)

- https://cert-portal.siemens.com/productcert/html/ssa-019113.html (ADP: siemens-SADP)

- https://cert-portal.siemens.com/productcert/html/ssa-457936.html (ADP: siemens-SADP)

## Provenance

- CNA: Linux: https://www.cve.org/CVERecord?id=CVE-2026-31555; retrieved 2026-11-05; source updated 2026-11-05

- FIRST EPSS: https://api.first.org/data/v1/epss?cve=CVE-2026-31555; retrieved 2026-11-05; source updated Not reported

Revision: 412eb247cd9bc19ba878c575addd0bd518b55e066f817e6c50ea72a9bdc1bcaf

Canonical: https://y2.dev/cve/CVE-2026-31555

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