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

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Date:   Sat, 29 Oct 2016 12:07:07 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Christoph Hellwig <hch@....de>, Jan Kara <jack@...e.cz>,
        Dmitry Monakhov <dmonakhov@...nvz.org>,
        Jeff Moyer <jmoyer@...hat.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-aio@...ck.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        stable <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 11:52 AM, Al Viro <viro@...iv.linux.org.uk> wrote:
>
> And that call can happen as soon as we return from __blockdev_direct_IO()
> (even earlier, actually).  As soon as that happens, the reference to
> struct file we'd acquired in io_submit_one() is dropped.  If descriptor
> table had been shared, another thread might have already closed that sucker,
> and fput() from aio_complete() would free struct file.

But that's the point. We don't *do* anything like that any more. We
now always do the final access from aio_complete(). So it doesn't
matter if that is called asynchronously (very early) or not.

That's the whole point of the patch. Exactly to do everything either
*before* we even submit it (at which point no completion can happen),
or doing it in aio_complete() which is guaranteed to be after the
submission. No races, no use-after-free.

What am I missing?

             Linus