linux-kernel - Re: [PATCH] sched_ext: Fix NULL pointer dereferences in put_prev_task

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Message-ID: <aEbO3DmwY4Tg6HT1@gpd4>
Date: Mon, 9 Jun 2025 14:09:00 +0200
From: Andrea Righi <arighi@...dia.com>
To: liuwenfang <liuwenfang@...or.com>
Cc: 'Tejun Heo' <tj@...nel.org>, 'David Vernet' <void@...ifault.com>,
	'Changwoo Min' <changwoo@...lia.com>,
	'Ingo Molnar' <mingo@...hat.com>,
	'Peter Zijlstra' <peterz@...radead.org>,
	'Juri Lelli' <juri.lelli@...hat.com>,
	'Vincent Guittot' <vincent.guittot@...aro.org>,
	'Dietmar Eggemann' <dietmar.eggemann@....com>,
	'Steven Rostedt' <rostedt@...dmis.org>,
	'Ben Segall' <bsegall@...gle.com>, 'Mel Gorman' <mgorman@...e.de>,
	'Valentin Schneider' <vschneid@...hat.com>,
	"'linux-kernel@...r.kernel.org'" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sched_ext: Fix NULL pointer dereferences in
 put_prev_task_scx

On Mon, Jun 09, 2025 at 11:36:15AM +0000, liuwenfang wrote:
> As put_prev_task can be used in other kernel modules which can lead
> to a NULL pointer. Fix this by checking for a valid next.

Actually, put_prev_task() should be used only within kernel/sched/ and, in
theory, you should have done a dequeue_task() before put_prev_task() in
this scenario, so SCX_TASK_QUEUED shouldn't be set in p->scx.flags.

The change might still make sense, but can you clarify how you triggered
the NULL pointer dereference?

Thanks,
-Andrea

> 
> Signed-off-by: l00013971 <l00013971@...onor.com>
> ---
>  kernel/sched/ext.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
> index f5133249f..6a579babd 100644
> --- a/kernel/sched/ext.c
> +++ b/kernel/sched/ext.c
> @@ -3262,7 +3262,7 @@ static void put_prev_task_scx(struct rq *rq, struct task_struct *p,
>  		 * ops.enqueue() that @p is the only one available for this cpu,
>  		 * which should trigger an explicit follow-up scheduling event.
>  		 */
> -		if (sched_class_above(&ext_sched_class, next->sched_class)) {
> +		if (next && sched_class_above(&ext_sched_class, next->sched_class)) {
>  			WARN_ON_ONCE(!static_branch_unlikely(&scx_ops_enq_last));
>  			do_enqueue_task(rq, p, SCX_ENQ_LAST, -1);
>  		} else {
> -- 
> 2.17.1