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

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Date:   Fri, 30 Apr 2021 09:40:48 +0000
From:   Quentin Perret <qperret@...gle.com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
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 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?

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)