CVE-2026-31555

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.

Affected Software

References

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