linux-kernel - Re: [PATCH] sched: fix potential use-after-free with cfs bandwidth

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Message-ID: <f35dbe78-1165-42ae-80fa-3465fec100ee@amd.com>
Date: Fri, 14 Feb 2025 09:37:50 +0530
From: K Prateek Nayak <kprateek.nayak@....com>
To: Josh Don <joshdon@...gle.com>, Ingo Molnar <mingo@...hat.com>, Peter
 Zijlstra <peterz@...radead.org>, Juri Lelli <juri.lelli@...hat.com>, Vincent
 Guittot <vincent.guittot@...aro.org>
CC: Dietmar Eggemann <dietmar.eggemann@....com>, Steven Rostedt
	<rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>, Mel Gorman
	<mgorman@...e.de>, Daniel Bristot de Oliveira <bristot@...hat.com>, Valentin
 Schneider <vschneid@...hat.com>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sched: fix potential use-after-free with cfs bandwidth

Hello Josh,

On 2/11/2025 1:21 AM, Josh Don wrote:
> We remove the cfs_rq throttled_csd_list entry *before* doing the
> unthrottle. The problem with that is that destroy_bandwidth() does a
> lockless scan of the system for any non-empty CSD lists. As a result,
> it is possible that destroy_bandwidth() returns while we still have a
> cfs_rq from the task group about to be unthrottled.
> 
> For full correctness, we should avoid removal from the list until after
> we're done unthrottling in __cfsb_csd_unthrottle().
> 
> For consistency, we make the same change to distribute_cfs_runtime(),
> even though this should already be safe due to destroy_bandwidth()
> cancelling the bandwidth hrtimers.
> 
> Signed-off-by: Josh Don <joshdon@...gle.com>

Other than a small nit: s/destroy_bandwidth/destroy_cfs_bandwidth/g
please feel free to add:

Reviewed-and-tested-by: K Prateek Nayak <kprateek.nayak@....com>

-- 
Thanks and Regards,
Prateek

> ---
>   kernel/sched/fair.c | 8 ++++----
>   1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 34fe6e9490c2..78f542ab03cf 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5917,10 +5917,10 @@ static void __cfsb_csd_unthrottle(void *arg)
>   
>   	list_for_each_entry_safe(cursor, tmp, &rq->cfsb_csd_list,
>   				 throttled_csd_list) {
> -		list_del_init(&cursor->throttled_csd_list);
> -
>   		if (cfs_rq_throttled(cursor))
>   			unthrottle_cfs_rq(cursor);
> +
> +		list_del_init(&cursor->throttled_csd_list);
>   	}
>   
>   	rcu_read_unlock();
> @@ -6034,11 +6034,11 @@ static bool distribute_cfs_runtime(struct cfs_bandwidth *cfs_b)
>   
>   		rq_lock_irqsave(rq, &rf);
>   
> -		list_del_init(&cfs_rq->throttled_csd_list);
> -
>   		if (cfs_rq_throttled(cfs_rq))
>   			unthrottle_cfs_rq(cfs_rq);
>   
> +		list_del_init(&cfs_rq->throttled_csd_list);
> +
>   		rq_unlock_irqrestore(rq, &rf);
>   	}
>   	SCHED_WARN_ON(!list_empty(&local_unthrottle));