2021-04-30 15:17:48

by Quentin Perret

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

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 <[email protected]>
Signed-off-by: Quentin Perret <[email protected]>
---
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

2021-04-30 15:30:57

by Vincent Guittot

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

On Fri, 30 Apr 2021 at 17:14, Quentin Perret <[email protected]> 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 <[email protected]>
> Signed-off-by: Quentin Perret <[email protected]>

Reviewed-by: Vincent Guittot <[email protected]>

> ---
> 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
>

2021-05-03 11:38:04

by Dietmar Eggemann

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

On 30/04/2021 17:27, Vincent Guittot wrote:
> On Fri, 30 Apr 2021 at 17:14, Quentin Perret <[email protected]> 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 <[email protected]>
>> Signed-off-by: Quentin Perret <[email protected]>
>
> Reviewed-by: Vincent Guittot <[email protected]>

I forgot that config UCLAMP_BUCKETS_COUNT is in range 5 ... 20.

So the error is bound to [-2 ... 5] (13/79, 20/51). I agree that we can
live with that.

Reviewed-by: Dietmar Eggemann <[email protected]>

>> ---
>> 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
>>

2021-05-03 18:09:09

by Peter Zijlstra

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

On Mon, May 03, 2021 at 12:55:34PM +0200, Dietmar Eggemann wrote:
> On 30/04/2021 17:27, Vincent Guittot wrote:
> > On Fri, 30 Apr 2021 at 17:14, Quentin Perret <[email protected]> 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 <[email protected]>
> >> Signed-off-by: Quentin Perret <[email protected]>
> >
> > Reviewed-by: Vincent Guittot <[email protected]>

> Reviewed-by: Dietmar Eggemann <[email protected]>

Thanks!

2021-05-06 13:53:16

by tip-bot2 for Alexey Makhalov

Subject: [tip: sched/urgent] sched: Fix out-of-bound access in uclamp

The following commit has been merged into the sched/urgent branch of tip:

Commit-ID: 6d2f8909a5fabb73fe2a63918117943986c39b6c
Gitweb: https://git.kernel.org/tip/6d2f8909a5fabb73fe2a63918117943986c39b6c
Author: Quentin Perret <[email protected]>
AuthorDate: Fri, 30 Apr 2021 15:14:12
Committer: Peter Zijlstra <[email protected]>
CommitterDate: Thu, 06 May 2021 15:33:26 +02:00

sched: Fix out-of-bound access in uclamp

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 <[email protected]>
Signed-off-by: Quentin Perret <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Reviewed-by: Vincent Guittot <[email protected]>
Reviewed-by: Dietmar Eggemann <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]
---
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 9143163..5226cc2 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -938,7 +938,7 @@ 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)