linux-kernel - Re: request_queue use-after-free

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Date:	Thu, 19 Nov 2015 22:56:43 +0100
From:	Ilya Dryomov <idryomov@...il.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Christoph Hellwig <hch@....de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-fsdevel@...r.kernel.org,
	Ceph Development <ceph-devel@...r.kernel.org>
Subject: Re: request_queue use-after-free - inode_detach_wb()

On Thu, Nov 19, 2015 at 10:18 PM, Tejun Heo <tj@...nel.org> wrote:
> Hello, Ilya.
>
> On Thu, Nov 19, 2015 at 09:56:21PM +0100, Ilya Dryomov wrote:
>> > Yes, that's where *I* think we should be headed.  Stuff in lower
>> > layers should stick around while upper layer things are around
>>
>> I think the fundamental problem is the embedding of bdi in the queue.
>> The lifetime rules (or, rather, expectations) for the two seem to be
>> completely different and, while used together, they belong to different
>> subsystems.  Even if we find a way to fix this particular race, there
>> is a good chance someone will reintroduce it in the future, perhaps in
>> a more subtle way.
>
> You're right.  This is nasty.  Hmmm... the root problem is that beyond
> the last __blkdev_put() the bdev and disk don't really have anything
> to do with each other but the bdev is still pointing to it.  We are
> already guaranteeing that the underlying disk hangs around while there
> are bdevs associated with it.
>
> We already know that the bdev is idle once bd_openers hits zero and
> the inode gets flushed, so at that point, the problem is bdev's
> inode->i_wb is still pointing to something that the bdev doesn't have
> anything to do with.  So, can we do inode_detach_wb() after flushing
> the inode?

Detaching the inode earlier is what I suggested in the first email, but
I didn't know if this kind of special casing was OK.  I'll try it out.

Thanks,

                Ilya
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