linux-kernel - Re: [PATCH] sched/psi: fix use-after-free in ep_remove_wait

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Date:   Wed, 1 Feb 2023 20:56:55 -0800
From:   Eric Biggers <ebiggers@...nel.org>
To:     Munehisa Kamata <kamatam@...zon.com>
Cc:     surenb@...gle.com, hannes@...xchg.org, hdanton@...a.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        mengcc@...zon.com, stable@...r.kernel.org
Subject: Re: [PATCH] sched/psi: fix use-after-free in ep_remove_wait_queue()

On Wed, Feb 01, 2023 at 07:00:23PM -0800, Munehisa Kamata wrote:
> diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
> index 8ac8b81bfee6..6e66c15f6450 100644
> --- a/kernel/sched/psi.c
> +++ b/kernel/sched/psi.c
> @@ -1343,10 +1343,11 @@ void psi_trigger_destroy(struct psi_trigger *t)
>  
>  	group = t->group;
>  	/*
> -	 * Wakeup waiters to stop polling. Can happen if cgroup is deleted
> -	 * from under a polling process.
> +	 * Wakeup waiters to stop polling and clear the queue to prevent it from
> +	 * being accessed later. Can happen if cgroup is deleted from under a
> +	 * polling process otherwise.
>  	 */
> -	wake_up_interruptible(&t->event_wait);
> +	wake_up_pollfree(&t->event_wait);
>  
>  	mutex_lock(&group->trigger_lock);

wake_up_pollfree() should only be used in extremely rare cases.  Why can't the
lifetime of the waitqueue be fixed instead?

- Eric