linux-kernel - Re: [PATCH v3] kernel/watch_queue: Make pipe NULL while clearing watch

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Message-ID: <Yun1rC59USrgd0fu@sol.localdomain>
Date:   Tue, 2 Aug 2022 21:12:28 -0700
From:   Eric Biggers <ebiggers@...nel.org>
To:     Siddh Raman Pant <code@...dh.me>
Cc:     David Howells <dhowells@...hat.com>,
        Christophe JAILLET <christophe.jaillet@...adoo.fr>,
        Eric Dumazet <edumazet@...gle.com>,
        "Fabio M. De Francesco" <fmdefrancesco@...il.com>,
        linux-security-modules <linux-security-module@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-kernel-mentees 
        <linux-kernel-mentees@...ts.linuxfoundation.org>,
        syzbot+c70d87ac1d001f29a058 
        <syzbot+c70d87ac1d001f29a058@...kaller.appspotmail.com>
Subject: Re: [PATCH v3] kernel/watch_queue: Make pipe NULL while clearing
 watch_queue

On Wed, Aug 03, 2022 at 09:26:04AM +0530, Siddh Raman Pant wrote:
> On Wed, 03 Aug 2022 06:38:36 +0530  Eric Biggers <ebiggers@...nel.org> wrote:
> > On Thu, Jul 28, 2022 at 09:21:21PM +0530, Siddh Raman Pant wrote:
> > > If not done, a reference to a freed pipe remains in the watch_queue,
> > > as this function is called before freeing a pipe in free_pipe_info()
> > > (see line 834 of fs/pipe.c).
> > > 
> > > This causes a UAF when post_one_notification() tries to access the pipe
> > > on a key update, which is reported by syzbot.
> > > 
> > > We also need to use READ_ONCE() in post_one_notification() to prevent the
> > > compiler from optimising and loading a non-NULL value from wqueue->pipe.
> > 
> > Didn't this already get fixed by the following commit?
> > 
> >     commit 353f7988dd8413c47718f7ca79c030b6fb62cfe5
> >     Author: Linus Torvalds <torvalds@...ux-foundation.org>
> >     Date:   Tue Jul 19 11:09:01 2022 -0700
> > 
> >         watchqueue: make sure to serialize 'wqueue->defunct' properly
> > 
> > With that, post_one_notification() only runs while the watch_queue is locked and
> > not "defunct".  So it's guaranteed that the pipe still exists.  Any concurrent
> > free_pipe_info() waits for the watch_queue to be unlocked in watch_queue_clear()
> > before proceeding to free the pipe.  So where is there still a bug?
> 
> It doesn't fix the dangling pointer to the freed pipe in the watch_queue, which
> had caused this crash.
> 

Under what circumstances is the pipe pointer still being dereferenced after the
pipe has been freed?  I don't see how it can be; see my explanation above.

- Eric