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

[Quentin Perret <qperret@google.com>]

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>

---

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