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Message-ID: <306d760d-fe6d-d02c-ad6c-e2467d8da4af@redhat.com>
Date:   Wed, 14 Dec 2022 11:55:24 -0500
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Subject: Re: [PATCH-block v3 1/2] bdi, blk-cgroup: Fix potential UAF of blkcg
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To:     Jens Axboe <axboe@kernel.dk>, Tejun Heo <tj@kernel.org>
Cc:     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,
        =?UTF-8?Q?Michal_Koutn=c3=bd?= <mkoutny@suse.com>,
        "Dennis Zhou (Facebook)" <dennisszhou@gmail.com>,
        Yi Zhang <yi.zhang@redhat.com>
References: <20221213184446.50181-1-longman@redhat.com>
 <20221213184446.50181-2-longman@redhat.com>
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 <5fbaea42-14a7-27a8-cea1-3a59161ceba0@kernel.dk>
From:   Waiman Long <longman@redhat.com>
In-Reply-To: <5fbaea42-14a7-27a8-cea1-3a59161ceba0@kernel.dk>
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On 12/14/22 11:54, Jens Axboe wrote:
> On 12/13/22 12:53 PM, Waiman Long wrote:
>> On 12/13/22 14:29, Tejun Heo wrote:
>>> 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>
>> Sorry, I overlooked the commit log in my update. I will update it if I need another version, or Jens can make the following edit:
>>
>> css_tryget() -> percpu_ref_is_zero().
> Since the other one also needs an edit, would be great if you could
> just send out a v4.
>
Sure, will do that.

Cheers,
Longman