Re: Re: NULL pointer dereference in pick_next_task_fair

[Kirill Tkhai]

On 06.11.2019 15:05, Peter Zijlstra wrote:
> On Mon, Oct 28, 2019 at 05:46:03PM +0000, Quentin Perret wrote:
>>
>> After digging a bit, the offending commit seems to be:
>>
>>     67692435c411 ("sched: Rework pick_next_task() slow-path")
>>
>> By 'offending' I mean that reverting it makes the issue go away. The
>> issue comes from the fact that pick_next_entity() returns a NULL se in
>> the 'simple' path of pick_next_task_fair(), which causes obvious
>> problems in the subsequent call to set_next_entity().
>>
>> I'll dig more, but if anybody understands the issue in the meatime feel
>> free to send me a patch to try out :)
> 
> So for all those who didn't follow along on IRC, the below seems to cure
> things.
> 
> The only thing I'm now considering is if we shouldn't be setting
> ->on_cpu=2 _before_ calling put_prev_task(). I'll go audit the RT/DL
> cases.
> 
> ---
> Subject: sched: Fix pick_next_task() vs 'change' pattern race
> From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Date: Mon Nov 4 22:18:14 CET 2019
> 
> Commit 67692435c411 ("sched: Rework pick_next_task() slow-path")
> inadvertly introduced a race because it changed a previously
> unexplored dependency between dropping the rq->lock and
> sched_class::put_prev_task().
> 
> The comments about dropping rq->lock, in for example
> newidle_balance(), only mentions the task being current and ->on_cpu
> being set. But when we look at the 'change' pattern (in for example
> sched_setnuma()):
> 
> 	queued = task_on_rq_queued(p); /* p->on_rq == TASK_ON_RQ_QUEUED */
> 	running = task_current(rq, p); /* rq->curr == p */
> 
> 	if (queued)
> 		dequeue_task(...);
> 	if (running)
> 		put_prev_task(...);
> 
> 	/* change task properties */
> 
> 	if (queued)
> 		enqueue_task(...);
> 	if (running)
> 		set_next_task(...);
> 
> It becomes obvious that if we do this after put_prev_task() has
> already been called on @p, things go sideways. This is exactly what
> the commit in question allows to happen when it does:
> 
> 	prev->sched_class->put_prev_task(rq, prev, rf);
> 	if (!rq->nr_running)
> 		newidle_balance(rq, rf);
> 
> The newidle_balance() call will drop rq->lock after we've called
> put_prev_task() and that allows the above 'change' pattern to
> interleave and mess up the state.
> 
> The order in pick_next_task() is mandated by the fact that RT/DL
> put_prev_task() can pull other RT tasks, in which case we should not
> call newidle_balance() since we'll not be going idle. Similarly, we
> cannot put newidle_balance() in put_prev_task_fair() because it should
> be called every time we'll end up selecting the idle task.
> 
> Given that we're stuck with this order, the only solution is fixing
> the 'change' pattern. The simplest fix seems to be to 'absuse'
> p->on_cpu to carry more state. Adding more state to p->on_rq is
> possible but is far more invasive and also ends up duplicating much of
> the state we already carry in p->on_cpu.
> 
> Introduce task_on_rq_curr() to indicate the if
> sched_class::set_next_task() has been called -- and we thus need to
> call put_prev_task().
> 
> Fixes: 67692435c411 ("sched: Rework pick_next_task() slow-path")
> Reported-by: Quentin Perret <qperret@xxxxxxxxxx>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> Tested-by: Quentin Perret <qperret@xxxxxxxxxx>
> Tested-by: Qais Yousef <qais.yousef@xxxxxxx>
> Tested-by: Valentin Schneider <valentin.schneider@xxxxxxx>
> Tested-by: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
> ---
>  kernel/sched/core.c  |   22 +++++++++++++++-------
>  kernel/sched/sched.h |   16 ++++++++++++++++
>  2 files changed, 31 insertions(+), 7 deletions(-)
> 
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1595,7 +1595,7 @@ void do_set_cpus_allowed(struct task_str
>  	lockdep_assert_held(&p->pi_lock);
>  
>  	queued = task_on_rq_queued(p);
> -	running = task_current(rq, p);
> +	running = task_on_rq_curr(rq, p);
>  
>  	if (queued) {
>  		/*
> @@ -3934,8 +3934,16 @@ pick_next_task(struct rq *rq, struct tas
>  	 * can PULL higher prio tasks when we lower the RQ 'priority'.
>  	 */
>  	prev->sched_class->put_prev_task(rq, prev, rf);
> -	if (!rq->nr_running)
> +#ifdef CONFIG_SMP
> +	if (!rq->nr_running) {
> +		/*
> +		 * Make sure task_on_rq_curr() fails, such that we don't do
> +		 * put_prev_task() + set_next_task() on this task again.
> +		 */
> +		prev->on_cpu = 2;
>  		newidle_balance(rq, rf);

Shouldn't we restore prev->on_cpu = 1 after newidle_balance()? Can't prev
become pickable again after newidle_balance() releases rq->lock, and we
take it again, so this on_cpu == 2 never will be cleared?

> +	}
> +#endif
>  
>  	for_each_class(class) {
>  		p = class->pick_next_task(rq, NULL, NULL);
> @@ -4422,7 +4430,7 @@ void rt_mutex_setprio(struct task_struct
>  
>  	prev_class = p->sched_class;
>  	queued = task_on_rq_queued(p);
> -	running = task_current(rq, p);
> +	running = task_on_rq_curr(rq, p);
>  	if (queued)
>  		dequeue_task(rq, p, queue_flag);
>  	if (running)
> @@ -4509,7 +4517,7 @@ void set_user_nice(struct task_struct *p
>  		goto out_unlock;
>  	}
>  	queued = task_on_rq_queued(p);
> -	running = task_current(rq, p);
> +	running = task_on_rq_curr(rq, p);
>  	if (queued)
>  		dequeue_task(rq, p, DEQUEUE_SAVE | DEQUEUE_NOCLOCK);
>  	if (running)
> @@ -4957,7 +4965,7 @@ static int __sched_setscheduler(struct t
>  	}
>  
>  	queued = task_on_rq_queued(p);
> -	running = task_current(rq, p);
> +	running = task_on_rq_curr(rq, p);
>  	if (queued)
>  		dequeue_task(rq, p, queue_flags);
>  	if (running)
> @@ -6141,7 +6149,7 @@ void sched_setnuma(struct task_struct *p
>  
>  	rq = task_rq_lock(p, &rf);
>  	queued = task_on_rq_queued(p);
> -	running = task_current(rq, p);
> +	running = task_on_rq_curr(rq, p);
>  
>  	if (queued)
>  		dequeue_task(rq, p, DEQUEUE_SAVE);
> @@ -7031,7 +7039,7 @@ void sched_move_task(struct task_struct
>  	rq = task_rq_lock(tsk, &rf);
>  	update_rq_clock(rq);
>  
> -	running = task_current(rq, tsk);
> +	running = task_on_rq_curr(rq, tsk);
>  	queued = task_on_rq_queued(tsk);
>  
>  	if (queued)
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1628,6 +1628,22 @@ static inline int task_running(struct rq
>  #endif
>  }
>  
> +/*
> + * If true, @p has had sched_class::set_next_task() called on it.
> + * See pick_next_task().
> + */
> +static inline bool task_on_rq_curr(struct rq *rq, struct task_struct *p)
> +{
> +#ifdef CONFIG_SMP
> +	return rq->curr == p && p->on_cpu == 1;
> +#else
> +	return rq->curr == p;
> +#endif
> +}
> +
> +/*
> + * If true, @p has has sched_class::enqueue_task() called on it.
> + */
>  static inline int task_on_rq_queued(struct task_struct *p)
>  {
>  	return p->on_rq == TASK_ON_RQ_QUEUED;
>