Re: [PATCH 2/2] futex/requeue: Prevent rcuwait use-after-free during requeue PI

[Sebastian Andrzej Siewior]

On 2026-07-20 10:50:55 [+0800], Yao Kai wrote:
> 
> 
> On 7/17/2026 5:38 PM, Sebastian Andrzej Siewior wrote:
> > On 2026-07-17 16:49:22 [+0800], Yao Kai wrote:
> > > On PREEMPT_RT, FUTEX_CMP_REQUEUE_PI can trigger a KASAN report:
> > > 
> > >    BUG: KASAN: slab-out-of-bounds in _raw_spin_lock_irqsave+0x76/0xe0
> > >    Call Trace:
> > >     _raw_spin_lock_irqsave+0x76/0xe0
> > >     try_to_wake_up+0xab/0x1540
> > >     rcuwait_wake_up+0x39/0x60
> > >     futex_requeue+0x18c3/0x1e10
> > > 
> > > The futex_q used by futex_wait_requeue_pi() is allocated on the waiter's
> > > stack. An early wakeup can race with a PI requeue as follows:
> > > 
> > >          waiter                          requeue task
> > >          ------                          ------------
> > > futex_wait_requeue_pi()
> > >    futex_do_wait()
> > >      schedule()
> > > 
> > >          * timeout/signal wakes waiter *
> > > 
> > >    futex_requeue_pi_wakeup_sync()
> > >      IN_PROGRESS -> WAIT
> > >      rcuwait_wait_event()
> > >                                          requeue_pi_wake_futex()
> > >                                            task = READ_ONCE(q->task)
> > >                                            futex_requeue_pi_complete()
> > >                                              WAIT -> LOCKED
> > >      return LOCKED
> > >    return
> > > // q lifetime ends
> > >                                              rcuwait_wake_up()
> > > 
> > > futex_requeue_pi_complete() publishes LOCKED before calling
> > > rcuwait_wake_up(). Once the waiter observes LOCKED, it can return from
> > > futex_wait_requeue_pi() and let q go out of scope before rcuwait_wake_up()
> > > reads q->requeue_wait.task and passes the stale pointer to
> > > try_to_wake_up().
> > > 
> > > Skip rcuwait_wake_up() for Q_REQUEUE_PI_LOCKED. requeue_pi_wake_futex()
> > > already saves q->task before publishing LOCKED and wakes the saved task
> > > afterward.
> > > 
> > > Fixes: 07d91ef510fb1 ("futex: Prevent requeue_pi() lock nesting issue on RT")
> > > Cc: stable@xxxxxxxxxxxxxxx
> > > Signed-off-by: Yao Kai <yaokai34@xxxxxxxxxx>
> > > ---
> > >   kernel/futex/requeue.c | 9 +++++++--
> > >   1 file changed, 7 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/kernel/futex/requeue.c b/kernel/futex/requeue.c
> > > index abc652b5b2dd..59e587775d9b 100644
> > > --- a/kernel/futex/requeue.c
> > > +++ b/kernel/futex/requeue.c
> > > @@ -155,8 +155,13 @@ static inline void futex_requeue_pi_complete(struct futex_q *q, int locked)
> > >   	} while (!atomic_try_cmpxchg(&q->requeue_state, &old, new));
> > >   #ifdef CONFIG_PREEMPT_RT
> > > -	/* If the waiter interleaved with the requeue let it know */
> > > -	if (unlikely(old == Q_REQUEUE_PI_WAIT))
> > > +	/*
> > > +	 * If the waiter interleaved with the requeue, let it know. For LOCKED,
> > > +	 * q may already be invalid, so requeue_pi_wake_futex() wakes the saved
> > > +	 * task instead.
> > > +	 */
> > > +	if (unlikely(old == Q_REQUEUE_PI_WAIT) &&
> > > +	    new != Q_REQUEUE_PI_LOCKED)
> > >   		rcuwait_wake_up(&q->requeue_wait);
> > 
> > Your whole assumption is based on the requeue_state in
> > futex_requeue_pi_wakeup_sync() changes from Q_REQUEUE_PI_IN_PROGRESS to
> > Q_REQUEUE_PI_WAIT and the rcuwait_wait_event() does not wait because the
> > condition becomes true before that happens. So the rcuwait_wake_up()
> > could access q.requeue_wait which is allocated on behalf of the waiter
> > which is gone. Certainly possible. But if we skip the wait in thise case
> > we probably miss the 99% cases where the waiter did wait, no?
> > 
> > This looks similar to commit b549113738e8c ("futex: Prevent
> > use-after-free during requeue-PI").
> > 
> > >   #endif
> > >   }
> > 
> > Sebastian
> 
> No wakeup is missed. Q_REQUEUE_PI_LOCKED is only published by
> requeue_pi_wake_futex(), which saves q->task before publishing the state
> and calls wake_up_state(task, TASK_NORMAL) afterwards. The other
> futex_requeue_pi_complete() callers produce DONE or an error state, and
> the rcuwait wakeup is retained for those paths.

 T1                                         T2
futex_requeue_pi_wakeup_sync()
  old = Q_REQUEUE_PI_IN_PROGRESS
  new = Q_REQUEUE_PI_WAIT
  cmpxchg()
  if (old == Q_REQUEUE_PI_IN_PROGRESS)
                                           futex_requeue_pi_complete
					     old = Q_REQUEUE_PI_WAIT
					     new = Q_REQUEUE_PI_LOCKED
					     cmpxchg()
   rcuwait_wait_event()
  <leave>
                                             rcuwait_wake_up(&q->requeue_wait);

So this your case then T2 updates the state before T1 enters sleep
because the condition was true before that. So you patch makes sense.
However given the more common case:

 T1                                         T2
futex_requeue_pi_wakeup_sync()
  old = Q_REQUEUE_PI_IN_PROGRESS
  new = Q_REQUEUE_PI_WAIT
  cmpxchg()
                                           futex_requeue_pi_complete
					     old = Q_REQUEUE_PI_WAIT
					     new = Q_REQUEUE_PI_LOCKED
					     cmpxchg()
  if (old == Q_REQUEUE_PI_IN_PROGRESS)
   rcuwait_wait_event()
                                             rcuwait_wake_up(&q->requeue_wait);
  <leave>

You will miss to wake T1 if T2 skips the wake, as suggested. Or do I
miss something?

> Thanks,
> Yao

Sebastian