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

[Sebastian Andrzej Siewior]

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