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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.

Fixes: 3ef240eaff ("futex: Prevent exit livelock")
Signed-off-by: Davidlohr Bueso <dave@stgolabs.net>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260326001759.4129680-1-dave@stgolabs.net
This commit is contained in:
Davidlohr Bueso 2026-03-25 17:17:59 -07:00 committed by Thomas Gleixner
parent 190a8c48ff
commit 210d36d892
1 changed files with 2 additions and 1 deletions
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3
kernel/futex/pi.c
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@ -918,7 +918,7 @@ int fixup_pi_owner(u32 __user *uaddr, struct futex_q *q, int locked)
int futex_lock_pi(u32 __user *uaddr, unsigned int flags, ktime_t *time, int trylock)
{
struct hrtimer_sleeper timeout, *to;
struct task_struct *exiting = NULL;
struct task_struct *exiting;
struct rt_mutex_waiter rt_waiter;
struct futex_q q = futex_q_init;
DEFINE_WAKE_Q(wake_q);
@ -933,6 +933,7 @@ int futex_lock_pi(u32 __user *uaddr, unsigned int flags, ktime_t *time, int tryl
to = futex_setup_timer(time, &timeout, flags, 0);
retry:
exiting = NULL;
ret = get_futex_key(uaddr, flags, &q.key, FUTEX_WRITE);
if (unlikely(ret != 0))
goto out;