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

[Chengming Zhou <chengming.zhou@linux.dev>]

On 2025/2/11 03:51, 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@google.com>

Good catch!

Reviewed-by: Chengming Zhou <chengming.zhou@linux.dev>

BTW, I just drew the cfs_rq UAF as below:

CPU0                            CPU1
__cfsb_csd_unthrottle()
     rq lock
     for each cfs_rq on list
         list_del_init from list
                                 unregister_fair_sched_group()
                                     destroy_cfs_bandwidth()
                                         if (list_empty(&rq->cfsb_csd_list))
                                             continue; // skip rq0
                                     if (cfs_rq->on_list) // maybe false
     	unthrottle_cfs_rq()
             add cfs_rq to list
     rq unlock
                                 cfs_rq freed after RCU grace period
cfs_rq UAF!

Thanks!

> ---
>   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));