linux-kernel - Re: [PATCH] aio: fix a user triggered use after free (and fix freeze protection of aio writes)

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Message-ID: <20161029152017.GA7388@lst.de>
Date:   Sat, 29 Oct 2016 17:20:17 +0200
From:   Christoph Hellwig <hch@....de>
To:     Al Viro <viro@...IV.linux.org.uk>
Cc:     Christoph Hellwig <hch@....de>, torvalds@...ux-foundation.org,
        jack@...e.cz, dmonakhov@...nvz.org, jmoyer@...hat.com,
        linux-fsdevel@...r.kernel.org, linux-aio@...ck.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH] aio: fix a user triggered use after free (and fix
        freeze protection of aio writes)

On Sat, Oct 29, 2016 at 01:24:51PM +0100, Al Viro wrote:
> How about taking this chunk (i.e. telling lockdep that we are not holding this
> thing) past the iter_op() call, where file_end_write() used to be?

We can't as that would not fix the use after free (at least for the lockdep
case - otherwise the call is a no-op).  Once iter_op returns aio_complete
might have dropped our reference to the file, and another thread might
have closed the fd so that the fput from aio_complete was the last one.

This is something that xfstests/323 can reproduce under the right conditions.