Re: [PATCH v6] psi: fix race between psi_trigger_create/destroy

[Suren Baghdasaryan]

On Thu, May 20, 2021 at 7:07 PM Huangzhaoyang <huangzhaoyang@xxxxxxxxx> wrote:
>
> From: Zhaoyang Huang <zhaoyang.huang@xxxxxxxxxx>
>
> Race detected between psi_trigger_destroy/create as shown below, which
> cause panic by accessing invalid psi_system->poll_wait->wait_queue_entry
> and psi_system->poll_timer->entry->next. Under this modification, the
> race window is removed by initialising poll_wait and poll_timer in
> group_init which are executed only once at beginning.
>
> psi_trigger_destroy                      psi_trigger_create
> mutex_lock(trigger_lock);
> rcu_assign_pointer(poll_task, NULL);
> mutex_unlock(trigger_lock);
>                                         mutex_lock(trigger_lock);
>                                         if (!rcu_access_pointer(group->poll_task)) {
>
>                                                 timer_setup(poll_timer, poll_timer_fn, 0);
>
>                                                 rcu_assign_pointer(poll_task, task);
>                                         }
>                                         mutex_unlock(trigger_lock);
>
> synchronize_rcu();
> del_timer_sync(poll_timer); <-- poll_timer has been reinitialized by
> psi_trigger_create
>
> So, trigger_lock/RCU correctly protects destruction of group->poll_task but
> misses this race affecting poll_timer and poll_wait.
>
> Fixes: 461daba06bdc ("psi: eliminate kthread_worker from psi trigger
> scheduling mechanism")
>
> Signed-off-by: ziwei.dai <ziwei.dai@xxxxxxxxxx>
> Signed-off-by: ke.wang <ke.wang@xxxxxxxxxx>
> Signed-off-by: Zhaoyang Huang <zhaoyang.huang@xxxxxxxxxx>

Looks good. Thanks!
Reviewed-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>

> ---
> v2: change del_timer_sync to del_timer in psi_trigger_destroy
> v3: remove timer_setup within psi_tirgger_create
>     protect del_timer by extending the critical section of mutex_lock
> v4: amend fix information on comment
> v5: delete the poll_timer while assigning the task to NULL
> v6: update subject and comments as Suren's suggestion
> ---
> ---
>  kernel/sched/psi.c | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
>
> diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
> index cc25a3c..58b36d1 100644
> --- a/kernel/sched/psi.c
> +++ b/kernel/sched/psi.c
> @@ -182,6 +182,8 @@ struct psi_group psi_system = {
>
>  static void psi_avgs_work(struct work_struct *work);
>
> +static void poll_timer_fn(struct timer_list *t);
> +
>  static void group_init(struct psi_group *group)
>  {
>         int cpu;
> @@ -201,6 +203,8 @@ static void group_init(struct psi_group *group)
>         memset(group->polling_total, 0, sizeof(group->polling_total));
>         group->polling_next_update = ULLONG_MAX;
>         group->polling_until = 0;
> +       init_waitqueue_head(&group->poll_wait);
> +       timer_setup(&group->poll_timer, poll_timer_fn, 0);
>         rcu_assign_pointer(group->poll_task, NULL);
>  }
>
> @@ -1157,9 +1161,7 @@ struct psi_trigger *psi_trigger_create(struct psi_group *group,
>                         return ERR_CAST(task);
>                 }
>                 atomic_set(&group->poll_wakeup, 0);
> -               init_waitqueue_head(&group->poll_wait);
>                 wake_up_process(task);
> -               timer_setup(&group->poll_timer, poll_timer_fn, 0);
>                 rcu_assign_pointer(group->poll_task, task);
>         }
>
> @@ -1211,6 +1213,7 @@ static void psi_trigger_destroy(struct kref *ref)
>                                         group->poll_task,
>                                         lockdep_is_held(&group->trigger_lock));
>                         rcu_assign_pointer(group->poll_task, NULL);
> +                       del_timer(&group->poll_timer);
>                 }
>         }
>
> @@ -1223,17 +1226,14 @@ static void psi_trigger_destroy(struct kref *ref)
>          */
>         synchronize_rcu();
>         /*
> -        * Destroy the kworker after releasing trigger_lock to prevent a
> +        * Stop kthread 'psimon' after releasing trigger_lock to prevent a
>          * deadlock while waiting for psi_poll_work to acquire trigger_lock
>          */
>         if (task_to_destroy) {
>                 /*
>                  * After the RCU grace period has expired, the worker
>                  * can no longer be found through group->poll_task.
> -                * But it might have been already scheduled before
> -                * that - deschedule it cleanly before destroying it.
>                  */
> -               del_timer_sync(&group->poll_timer);
>                 kthread_stop(task_to_destroy);
>         }
>         kfree(t);
> --
> 1.9.1
>