Re: [PATCH v3] sched: Fix out-of-bound access in uclamp

[Vincent Guittot]

On Fri, 30 Apr 2021 at 17:14, Quentin Perret <qperret@xxxxxxxxxx> wrote:
>
> 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@xxxxxxx>
> Signed-off-by: Quentin Perret <qperret@xxxxxxxxxx>

Reviewed-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>

> ---
> 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
>