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

[Johannes Weiner]

On Fri, May 21, 2021 at 10:05:54AM +0800, Huangzhaoyang 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>

Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>