Re: [PATCH] sched/fair: Fix fault in reweight_entity

[Vincent Guittot]

On Wed, 19 Jan 2022 at 02:24, Tadeusz Struk <tadeusz.struk@xxxxxxxxxx> wrote:
>
> Syzbot found a GPF in reweight_entity. This has been bisected to commit
> c85c6fadbef0 ("kernel/sched: Fix sched_fork() access an invalid sched_task_group")
> Looks like after this change there is a time window, when
> task_struct->se.cfs_rq can be NULL. This can be exploited to trigger
> null-ptr-deref by calling setpriority on that task.
>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Juri Lelli <juri.lelli@xxxxxxxxxx>
> Cc: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> Cc: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Cc: Ben Segall <bsegall@xxxxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxx>
> Cc: Daniel Bristot de Oliveira <bristot@xxxxxxxxxx>
> Cc: Zhang Qiao <zhangqiao22@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
>
> Link: https://syzkaller.appspot.com/bug?id=9d9c27adc674e3a7932b22b61c79a02da82cbdc1
> Fixes: c85c6fadbef0 ("kernel/sched: Fix sched_fork() access an invalid sched_task_group")

The sha1 doesn't look correct.

> Signed-off-by: Tadeusz Struk <tadeusz.struk@xxxxxxxxxx>
> ---
>  kernel/sched/fair.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 095b0aa378df..196f8cee3f9b 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -3042,6 +3042,9 @@ dequeue_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *se) { }
>  static void reweight_entity(struct cfs_rq *cfs_rq, struct sched_entity *se,
>                             unsigned long weight)
>  {
> +       if (!cfs_rq)
> +               return;
> +
>         if (se->on_rq) {
>                 /* commit outstanding execution time */
>                 if (cfs_rq->curr == se)
> --
> 2.34.1
>