Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Message-ID: <34a8c4a7-a58d-63fc-4599-accf1cbb6aae@redhat.com>
Date:   Tue, 13 Dec 2022 14:53:52 -0500
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101
 Thunderbird/102.4.0
Subject: Re: [PATCH-block v3 1/2] bdi, blk-cgroup: Fix potential UAF of blkcg
Content-Language: en-US
To:     Tejun Heo <tj@kernel.org>
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,
        =?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>
 <Y5jSllwwBdmQ1jQz@slm.duckdns.org>
From:   Waiman Long <longman@redhat.com>
In-Reply-To: <Y5jSllwwBdmQ1jQz@slm.duckdns.org>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
Precedence: bulk


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().

Thanks,
Longman