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Sender: Tejun Heo <htejun@gmail.com>
Date:   Tue, 13 Dec 2022 09:29:26 -1000
From:   Tejun Heo <tj@kernel.org>
To:     Waiman Long <longman@redhat.com>
Cc:     Jens Axboe <axboe@kernel.dk>, Josef Bacik <josef@toxicpanda.com>,
        Zefan Li <lizefan.x@bytedance.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        cgroups@vger.kernel.org, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Michal =?iso-8859-1?Q?Koutn=FD?= <mkoutny@suse.com>,
        "Dennis Zhou (Facebook)" <dennisszhou@gmail.com>,
        Yi Zhang <yi.zhang@redhat.com>
Subject: Re: [PATCH-block v3 1/2] bdi, blk-cgroup: Fix potential UAF of blkcg
Message-ID: <Y5jSllwwBdmQ1jQz@slm.duckdns.org>
References: <20221213184446.50181-1-longman@redhat.com>
 <20221213184446.50181-2-longman@redhat.com>
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Precedence: bulk

On Tue, Dec 13, 2022 at 01:44:45PM -0500, Waiman Long wrote:
> Commit 59b57717fff8 ("blkcg: delay blkg destruction until after
> writeback has finished") delayed call to blkcg_destroy_blkgs() to
> cgwb_release_workfn(). However, it is done after a css_put() of blkcg
> which may be the final put that causes the blkcg to be freed as RCU
> read lock isn't held.
> 
> Another place where blkcg_destroy_blkgs() can be called indirectly via
> blkcg_unpin_online() is from the offline_css() function called from
> css_killed_work_fn(). Over there, the potentially final css_put() call
> is issued after offline_css().
> 
> By adding a css_tryget() into blkcg_destroy_blkgs() and warning its
> failure, the following stack trace was produced in a test system on
> bootup.

This doesn't agree with the code anymore. Otherwise

Acked-by: Tejun Heo <tj@kernel.org>

Thanks.

-- 
tejun