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: <20161030094437.GA3225@quack2.suse.cz>
Date:   Sun, 30 Oct 2016 10:44:37 +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 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...

								Honza 

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