Messages in this thread | | | Subject | Re: [PATCH] sched: Fix out-of-bound access in uclamp | From | Dietmar Eggemann <> | Date | Thu, 29 Apr 2021 14:34:14 +0200 |
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On 28/04/2021 19:27, Quentin Perret 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=51.2, > rounded to the closest value: 51. Now, a task with a clamp of 1024 (as > is the default for the min clamp of RT tasks) will be mapped to bucket > id 1024/51=20 as we're now using a standard integer division. Sadly, > correct indexes are in range [0,19], hence leading to an out of bound > memory access. > > Fix this by using a rounding-up division when computing the bucket size.
But in case you use e.g. 16 buckets, wouldn't you still end up with this task mapped into bucket_id=16?
1024/16=64
1024/64=16
> > 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> > > --- > > 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..ec175909e8b0 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 DIV_ROUND_UP(SCHED_CAPACITY_SCALE, UCLAMP_BUCKETS) > > #define for_each_clamp_id(clamp_id) \ > for ((clamp_id) = 0; (clamp_id) < UCLAMP_CNT; (clamp_id)++) >
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