linux-kernel - Re: [Kernel Bug] KASAN: slab-use-after-free Read in __blkcg_rstat

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Message-ID: <aYII17gzXXPCTS3p@fedora>
Date: Tue, 3 Feb 2026 22:40:23 +0800
From: Ming Lei <ming.lei@...hat.com>
To: Michal Koutný <mkoutny@...e.com>
Cc: 李龙兴 <coregee2000@...il.com>,
	syzkaller@...glegroups.com, tj@...nel.org, josef@...icpanda.com,
	axboe@...nel.dk, cgroups@...r.kernel.org,
	linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
	yukuai@...as.com
Subject: Re: [Kernel Bug] KASAN: slab-use-after-free Read in
 __blkcg_rstat_flush

On Tue, Feb 03, 2026 at 03:16:43PM +0100, Michal Koutný wrote:
> On Tue, Feb 03, 2026 at 10:08:07PM +0800, Ming Lei <ming.lei@...hat.com> wrote:
> > I can't parse your question, here blkg_release() simply needs to flush
> > all stats. Why do you talk about preventing new flush? why is it related
> > with this UAF?
> 
> What prevents this fix:
> 
> --- a/block/blk-cgroup.c
> +++ b/block/blk-cgroup.c
> @@ -169,14 +169,6 @@ static void __blkg_release(struct rcu_head *rcu)
>  #ifdef CONFIG_BLK_CGROUP_PUNT_BIO
>         WARN_ON(!bio_list_empty(&blkg->async_bios));
>  #endif
> -       /*
> -        * Flush all the non-empty percpu lockless lists before releasing
> -        * us, given these stat belongs to us.
> -        *
> -        * blkg_stat_lock is for serializing blkg stat update
> -        */
> -       for_each_possible_cpu(cpu)
> -               __blkcg_rstat_flush(blkcg, cpu);
> 
>         /* release the blkcg and parent blkg refs this blkg has been holding */
>         css_put(&blkg->blkcg->css);
> @@ -195,6 +187,15 @@ static void blkg_release(struct percpu_ref *ref)
>  {
>         struct blkcg_gq *blkg = container_of(ref, struct blkcg_gq, refcnt);
> 
> +       /*
> +        * Flush all the non-empty percpu lockless lists before releasing
> +        * us, given these stat belongs to us.
> +        *
> +        * blkg_stat_lock is for serializing blkg stat update
> +        */
> +       for_each_possible_cpu(cpu)
> +               __blkcg_rstat_flush(blkcg, cpu);
> +
>         call_rcu(&blkg->rcu_head, __blkg_release);
>  }

This one looks more clever, can you send one formal patch?


Thanks, 
Ming