[PATCH 5.15 397/570] futex: Clear stale exiting pointer in futex_lock_pi() retry path

Greg Kroah-Hartman
2026-04-13T16:35:12Z

From: Davidlohr Bueso <dave@stgolabs.net>

commit 210d36d892de5195e6766c45519dfb1e65f3eb83 upstream.

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: 3ef240eaff36 ("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
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
kernel/futex/core.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

--- a/kernel/futex/core.c
+++ b/kernel/futex/core.c
@@ -3029,9 +3029,9 @@ static int futex_lock_pi(u32 __user *uad
ktime_t *time, int trylock)
{
struct hrtimer_sleeper timeout, *to;
- struct task_struct *exiting = NULL;
struct rt_mutex_waiter rt_waiter;
struct futex_hash_bucket *hb;
+ struct task_struct *exiting;
struct futex_q q = futex_q_init;
int res, ret;

@@ -3044,6 +3044,7 @@ static int futex_lock_pi(u32 __user *uad
to = futex_setup_timer(time, &timeout, flags, 0);

retry:
+ exiting = NULL;
ret = get_futex_key(uaddr, flags & FLAGS_SHARED, &q.key, FUTEX_WRITE);
if (unlikely(ret != 0))
goto out;