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Message-ID: <20250324115637.26379f8c@gandalf.local.home>
Date: Mon, 24 Mar 2025 11:56:37 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: Pat Cody <patcody@...a.com>
Cc: <mingo@...hat.com>, <peterz@...radead.org>, <juri.lelli@...hat.com>,
<vincent.guittot@...aro.org>, <dietmar.eggemann@....com>,
<bsegall@...gle.com>, <mgorman@...e.de>, <vschneid@...hat.com>,
<linux-kernel@...r.kernel.org>, <riel@...riel.com>, <kernel-team@...a.com>,
<stable@...nel.org>
Subject: Re: [PATCH] sched/fair: Add null pointer check to
pick_next_entity()
On Thu, 20 Mar 2025 10:34:39 -0700
Pat Cody <patcody@...a.com> wrote:
> pick_eevdf() can return null, resulting in a null pointer dereference
> crash in pick_next_entity()
>
> The other call site of pick_eevdf() can already handle a null pointer,
> and pick_next_entity() can already return null as well. Add an extra
> check to handle the null return here.
Have you actually triggered this?
>
> Cc: stable@...nel.org
> Fixes: f12e148892ed ("sched/fair: Prepare pick_next_task() for delayed dequeue")
No signed-off-by.
> ---
> kernel/sched/fair.c | 2 ++
> 1 file changed, 2 insertions(+)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index a553181dc764..f2157298cbce 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5560,6 +5560,8 @@ pick_next_entity(struct rq *rq, struct cfs_rq *cfs_rq)
> }
>
> struct sched_entity *se = pick_eevdf(cfs_rq);
> + if (!se)
> + return NULL;
So the place that this is called is in pick_task_fair() which has:
cfs_rq = &rq->cfs;
if (!cfs_rq->nr_queued)
return NULL;
Where I believe it's a bug if cfs_rq->nr_queued > 0 and pick_eevdf() returns NULL.
Which means in should never happen. Now, to be more robust, we could add a
check for NULL, but this is not a fix, nor should it go to stable.
-- Steve
> if (se->sched_delayed) {
> dequeue_entities(rq, se, DEQUEUE_SLEEP | DEQUEUE_DELAYED);
> /*
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