Messages in this thread Patch in this message | | | Date | Thu, 29 Apr 2021 15:26:56 +0000 | Subject | [PATCH v2] sched: Fix out-of-bound access in uclamp | From | Quentin Perret <> |
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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.
Fix the math to compute the bucket size.
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>
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Changes in v2: - replaced the DIV_ROUND_UP(a,b) with a/b+1 (Dietmar)
This was found thanks to the SCHED_WARN_ON() in uclamp_rq_dec_id() which indicated a broken state while running with 20 buckets on Android.
Big thanks to Qais for the help with this one. --- kernel/sched/core.c | 3 +-- 1 file changed, 1 insertion(+), 2 deletions(-)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 98191218d891..c5fb230dc604 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -920,8 +920,7 @@ static struct uclamp_se uclamp_default[UCLAMP_CNT]; */ DEFINE_STATIC_KEY_FALSE(sched_uclamp_used); -/* Integer rounded range for each bucket */ -#define UCLAMP_BUCKET_DELTA DIV_ROUND_CLOSEST(SCHED_CAPACITY_SCALE, UCLAMP_BUCKETS) +#define UCLAMP_BUCKET_DELTA (SCHED_CAPACITY_SCALE / UCLAMP_BUCKETS + 1) #define for_each_clamp_id(clamp_id) \ for ((clamp_id) = 0; (clamp_id) < UCLAMP_CNT; (clamp_id)++) -- 2.31.1.498.g6c1eba8ee3d-goog
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