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From: Bart Van Assche <bvanassche@acm.org>
To: Ming Lei <ming.lei@redhat.com>
Cc: Christoph Hellwig <hch@infradead.org>,
	Jens Axboe <axboe@kernel.dk>,
	linux-block@vger.kernel.org, Hannes Reinecke <hare@suse.de>,
	John Garry <john.garry@huawei.com>,
	David Jeffery <djeffery@redhat.com>
Subject: Re: [PATCH V5 3/4] blk-mq: clear stale request in tags->rq[] before freeing one request pool
Date: Thu, 6 May 2021 18:10:09 -0700	[thread overview]
Message-ID: <739456b9-e8d4-310e-9bf3-7b8930a1e51c@acm.org> (raw)
In-Reply-To: <YJSFm99PUlLedF+D@T590>

On 5/6/21 5:11 PM, Ming Lei wrote:
> On Thu, May 06, 2021 at 07:51:53AM -0700, Bart Van Assche wrote:
>> I'm not sure that would be a better name since I don't think that it is
>> necessary to hold that lock around the cmpxchg() calls. How about using
>> something like the following code in blk_mq_clear_rq_mapping() instead
>> of the code in v5 of patch 3/4?
>>
>> 	spin_lock_irqsave(&drv_tags->lock, flags);
>> 	spin_unlock_irqrestore(&drv_tags->lock, flags);
>>
>> 	list_for_each_entry(page, &tags->page_list, lru) {
>> 		/* use cmpxchg() to clear request pointer selectively */
>> 	}
> 
> This way won't work as expected because iterating may happen between
> releasing drv_tags->lock and cmpxchg(->rqs[]), then freed requests
> may still be touched during iteration after they are freed in blk_mq_free_rqs().

Right, the unlock should happen after the pointers have been cleared. 
But I think it is safe to move the spin_lock call down such that both 
the spin_lock and spin_unlock call happen after the pointers have been 
cleared. That is sufficient to guarantee that all 
blk_mq_find_and_get_req() calls either happen before or after the spin 
lock / unlock pair. blk_mq_find_and_get_req() calls that happen before 
the pair happen before the request pointers are freed. Calls that happen 
after the spin lock / unlock pair will either read NULL or a pointer to 
a request that is associated with another queue and hence won't trigger 
a use-after-free.

Bart.

  reply	other threads:[~2021-05-07  1:10 UTC|newest]

Thread overview: 26+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-05-05 14:58 [PATCH V5 0/4] blk-mq: fix request UAF related with iterating over tagset requests Ming Lei
2021-05-05 14:58 ` [PATCH V5 1/4] block: avoid double io accounting for flush request Ming Lei
2021-05-06  6:44   ` Christoph Hellwig
2021-05-05 14:58 ` [PATCH V5 2/4] blk-mq: grab rq->refcount before calling ->fn in blk_mq_tagset_busy_iter Ming Lei
2021-05-06  6:54   ` Christoph Hellwig
2021-05-06  7:30     ` Ming Lei
2021-05-05 14:58 ` [PATCH V5 3/4] blk-mq: clear stale request in tags->rq[] before freeing one request pool Ming Lei
2021-05-06  7:12   ` Christoph Hellwig
2021-05-06  7:34     ` Ming Lei
2021-05-06 12:18       ` Christoph Hellwig
2021-05-06 13:38         ` Ming Lei
2021-05-07  6:57           ` Christoph Hellwig
2021-05-07  7:30             ` Ming Lei
2021-05-06 14:51         ` Bart Van Assche
2021-05-07  0:11           ` Ming Lei
2021-05-07  1:10             ` Bart Van Assche [this message]
2021-05-07  2:05               ` Ming Lei
2021-05-07  3:16                 ` Bart Van Assche
2021-05-07  6:31                   ` Ming Lei
2021-05-07  6:52                     ` Christoph Hellwig
2021-05-08  2:02                       ` Bart Van Assche
2021-05-06 15:02   ` Bart Van Assche
2021-05-07  0:13     ` Ming Lei
2021-05-07  1:11   ` Bart Van Assche
2021-05-07  2:06     ` Ming Lei
2021-05-05 14:58 ` [PATCH V5 4/4] blk-mq: clearing flush request reference in tags->rqs[] Ming Lei

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