linux-kernel - Re: [PATCH v2] sched: Fix out-of-bound access in uclamp

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Date:   Fri, 30 Apr 2021 14:03:54 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Quentin Perret <qperret@...gle.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Qais Yousef <qais.yousef@....com>,
        Android Kernel Team <kernel-team@...roid.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Patrick Bellasi <patrick.bellasi@...bug.net>
Subject: Re: [PATCH v2] sched: Fix out-of-bound access in uclamp

On Fri, 30 Apr 2021 at 11:40, Quentin Perret <qperret@...gle.com> wrote:
>
> On Friday 30 Apr 2021 at 10:49:50 (+0200), Vincent Guittot wrote:
> > 20 buckets is probably not the best example because of the rounding of
> > the division. With 16 buckets, each bucket should be exactly 64 steps
> > large except the last one which will have 65 steps because of the
> > value 1024. With your change, buckets will be 65 large and the last
> > one will be only 49 large
>
> OK, so what do you think of this?

Looks good to me


>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index c5fb230dc604..dceeb5821797 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -920,14 +920,14 @@ static struct uclamp_se uclamp_default[UCLAMP_CNT];
>   */
>  DEFINE_STATIC_KEY_FALSE(sched_uclamp_used);
>
> -#define UCLAMP_BUCKET_DELTA (SCHED_CAPACITY_SCALE / UCLAMP_BUCKETS + 1)
> +#define UCLAMP_BUCKET_DELTA DIV_ROUND_CLOSEST(SCHED_CAPACITY_SCALE, UCLAMP_BUCKETS)
>
>  #define for_each_clamp_id(clamp_id) \
>         for ((clamp_id) = 0; (clamp_id) < UCLAMP_CNT; (clamp_id)++)
>
>  static inline unsigned int uclamp_bucket_id(unsigned int clamp_value)
>  {
> -       return clamp_value / UCLAMP_BUCKET_DELTA;
> +       return min(clamp_value / UCLAMP_BUCKET_DELTA, UCLAMP_BUCKETS - 1);
>  }
>
>  static inline unsigned int uclamp_none(enum uclamp_id clamp_id)