linux-kernel - Re: [V2 PATCH 1/2] aio: clean up aio_migratepage() and related code much

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Message-ID: <20140324132058.GH4173@kvack.org>
Date:	Mon, 24 Mar 2014 09:20:58 -0400
From:	Benjamin LaHaise <bcrl@...ck.org>
To:	Gu Zheng <guz.fnst@...fujitsu.com>
Cc:	Tang Chen <tangchen@...fujitsu.com>, Dave Jones <davej@...hat.com>,
	Al Viro <viro@...iv.linux.org.uk>, jmoyer@...hat.com,
	kosaki.motohiro@...fujitsu.com,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>,
	miaox@...fujitsu.com, linux-aio@...ck.org,
	fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [V2 PATCH 1/2] aio: clean up aio_migratepage() and related code much

On Mon, Mar 24, 2014 at 06:59:30PM +0800, Gu Zheng wrote:
> As the page migration framework holds lock_page() to protect the pages
> (both old and new) while migrating, so while the page migrating, both
> of old page and new page are locked. And the aio context teardown
> routine will call *truncate*(in put_aio_ring_file()) to truncate
> pagecache which needs to acquire page_lock() for each page one by one.
> So there is a native mutual exclusion between *migrate page* v.s. truncate().
> 
> If put_aio_ring_file() is called at first of the context teardown flow
> (aio_free_ring). Then, page migration and ctx freeing will have mutual
> execution guarded by lock_page() v.s. truncate(). Once a page is removed
> from radix-tree, it will not be migrated. On the other hand, the context
> can not be freed while the page migraiton are ongoing.

Sorry, but your change to remove the taking of ->private_lock in 
put_aio_ring_file() is not safe.  If a malicious user reinstantiates 
any pages in the ring buffer's mmaping, there is nothing protecting 
the system against incoherent accesses of ->ring_pages.  One possible 
way of making this occur would be to use mremap() to expand the size 
of the mapping or move it to a different location in the user process' 
address space.  Yes, it's a tiny race, but it's possible.  There is 
absolutely no reason to remove this locking -- ring teardown is 
hardly a performance sensitive code path.  I'm going to stick with my 
approach instead.

		-ben
-- 
"Thought is the essence of where you are now."
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