linux-kernel - Re: [PATCH-block v2 1/3] bdi, blk-cgroup: Fix potential UAF of blkcg

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Date:   Mon, 12 Dec 2022 12:13:47 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Jens Axboe <axboe@...nel.dk>, Josef Bacik <josef@...icpanda.com>,
        Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        cgroups@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Michal Koutný <mkoutny@...e.com>,
        "Dennis Zhou (Facebook)" <dennisszhou@...il.com>,
        Yi Zhang <yi.zhang@...hat.com>
Subject: Re: [PATCH-block v2 1/3] bdi, blk-cgroup: Fix potential UAF of blkcg

On Sun, Dec 11, 2022 at 05:20:56PM -0500, Waiman Long wrote:
>  static void blkcg_destroy_blkgs(struct blkcg *blkcg)
>  {
> +	/*
> +	 * blkcg_destroy_blkgs() shouldn't be called with all the blkcg
> +	 * references gone.
> +	 */
> +	if (WARN_ON_ONCE(!css_tryget(&blkcg->css)))
> +		return;

Wouldn't it make more sense to use percpu_ref_is_zero()? It's not like the
obtained extra reference does anything, right?

Thanks.

-- 
tejun