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

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Date:   Sun, 30 Oct 2016 11:52:31 +0100
From:   Jan Kara <jack@...e.cz>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Al Viro <viro@...iv.linux.org.uk>, 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 Sun 30-10-16 10:44:37, Jan Kara wrote:
> On Sat 29-10-16 13:09:25, Linus Torvalds wrote:
> > On Sat, Oct 29, 2016 at 12:17 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
> > >
> > > AFAICS, the possibility of dropping the last reference to struct file
> > > before ->write_iter() has returned is fundamentally broken.  I might be
> > > missing something subtle here, but...
> > 
> > Ok, let's add a get_file(); fput(); around that whole iter call sequence.
> > 
> > And that's a separate issue from "we should hold the fs freezer lock
> > around the whole operation". So I think we need both.
> 
> Yup, these are two separate issues. I'm fine with adding get_file(); fput()
> around the iter call sequence. Then, when we have struct file available for
> the whole time ->write_iter runs, I'd prefer to keep the call to fool
> lockdep where original file_end_write() call was - that gives us proper
> lockdep coverage for all the code behind iter_op(). The downside is we
> cannot keep helpers as elegant as you suggested in your patch but I believe
> it's bearable and worth the additional lockdep coverage. I'll send patches
> shortly...

Hum, the additional refcount patch oopses on me when running generic/323,
I'll have to board to my flight to US shortly so I won't be able to send it
soon - maybe when I'm transferring in Denver ;).

								Honza
--
Jan Kara <jack@...e.com>
SUSE Labs, CR