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Date:   Fri, 30 Apr 2021 15:14:12 +0000
Message-Id: <20210430151412.160913-1-qperret@google.com>
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Subject: [PATCH v3] sched: Fix out-of-bound access in uclamp
From:   Quentin Perret <qperret@google.com>
To:     mingo@redhat.com, peterz@infradead.org, vincent.guittot@linaro.org,
        juri.lelli@redhat.com
Cc:     dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com,
        mgorman@suse.de, bristot@redhat.com, qais.yousef@arm.com,
        kernel-team@android.com, linux-kernel@vger.kernel.org,
        patrick.bellasi@matbug.net
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

Util-clamp places tasks in different buckets based on their clamp values
for performance reasons. However, the size of buckets is currently
computed using a rounding division, which can lead to an off-by-one
error in some configurations.

For instance, with 20 buckets, the bucket size will be 1024/20=51. A
task with a clamp of 1024 will be mapped to bucket id 1024/51=20. Sadly,
correct indexes are in range [0,19], hence leading to an out of bound
memory access.

Clamp the bucket id to fix the issue.

Fixes: 69842cba9ace ("sched/uclamp: Add CPU's clamp buckets refcounting")
Suggested-by: Qais Yousef <qais.yousef@arm.com>
Signed-off-by: Quentin Perret <qperret@google.com>
---
Changes in v3:
 - Keep rounding div to improve fairness (Vincent)
---
 kernel/sched/core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 98191218d891..c12ec648423e 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -928,7 +928,7 @@ DEFINE_STATIC_KEY_FALSE(sched_uclamp_used);
 
 static inline unsigned int uclamp_bucket_id(unsigned int clamp_value)
 {
-	return clamp_value / UCLAMP_BUCKET_DELTA;
+	return min_t(unsigned int, clamp_value / UCLAMP_BUCKET_DELTA, UCLAMP_BUCKETS - 1);
 }
 
 static inline unsigned int uclamp_none(enum uclamp_id clamp_id)
-- 
2.31.1.527.g47e6f16901-goog