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Message-ID: <d689d6d9-9206-028e-1363-8d7d77d55e79@redhat.com>
Date:   Wed, 30 Nov 2022 10:44:36 -0500
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Subject: Re: [PATCH-block v2] bdi, blk-cgroup: Fix potential UAF of blkcg
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To:     =?UTF-8?Q?Michal_Koutn=c3=bd?= <mkoutny@suse.com>
Cc:     Jens Axboe <axboe@kernel.dk>, Tejun Heo <tj@kernel.org>,
        cgroups@vger.kernel.org, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Ming Lei <ming.lei@redhat.com>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Hillf Danton <hdanton@sina.com>,
        Chaitanya Kulkarni <chaitanyak@nvidia.com>,
        Bart Van Assche <bvanassche@acm.org>,
        Josef Bacik <josef@toxicpanda.com>,
        Yi Zhang <yi.zhang@redhat.com>
References: <20221129203400.1456100-1-longman@redhat.com>
 <20221130151639.GE27838@blackbody.suse.cz>
From:   Waiman Long <longman@redhat.com>
In-Reply-To: <20221130151639.GE27838@blackbody.suse.cz>
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On 11/30/22 10:16, Michal Koutný wrote:
> On Tue, Nov 29, 2022 at 03:34:00PM -0500, Waiman Long <longman@redhat.com> wrote:
>> The reproducing system can no longer produce a warning with this patch.
>> All the runnable block/0* tests including block/027 were run successfully
>> without failure.
> Thanks for the test!
>
>> @@ -1088,7 +1088,15 @@ static void blkcg_destroy_blkgs(struct blkcg *blkcg)
>>   
>>   	might_sleep();
>>   
>> -	css_get(&blkcg->css);
>> +	/*
>> +	 * blkcg_destroy_blkgs() shouldn't be called with all the blkcg
>> +	 * references gone and rcu_read_lock not held.
>> +	 */
>> +	if (!css_tryget(&blkcg->css)) {
>> +		WARN_ON_ONCE(!rcu_read_lock_held());
>> +		return;
>> +	}
> As I followed the previous discussion, the principle is that obtaining a
> reference or being inside an RCU read section is sufficient.
>
> Consequently, I'd expect the two situations handled equally but here the
> no-ref but RCU bails out. (Which is OK because blkg_list must be empty?)
>
> However, the might_sleep() in (non-sleepable) RCU reader section combo
> makes me wary anyway (not with the early return but tools would likely
> complain).
>
> All in all, can't the contract of blkcg_destroy_blkgs() declare that
> a caller must pass blkcg with a valid reference? (The body of
> blkcg_destroy_blkgs then wouldn't need to get neither put the inner
> reference).

You are right. I should have pushed the might_sleep down(). Will post a 
new version to fix that.

Thanks,
Longman