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

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Date:   Tue, 2 Aug 2022 22:41:31 -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 10:43:31AM +0530, Siddh Raman Pant wrote:
> On Wed, 03 Aug 2022 09:42:28 +0530  Eric Biggers <ebiggers@...nel.org> wrote:
> > 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.
> 
> It really didn't fix the crash. It caused the same crash reported here, which
> I was already locally getting:
> https://syzkaller.appspot.com/bug?extid=03d7b43290037d1f87ca

I tested the syzbot reproducer
https://syzkaller.appspot.com/text?tag=ReproC&x=174ea97e080000, and it does
*not* trigger the bug on the latest upstream.  But, it does trigger the bug if I
recent Linus's recent watch_queue fixes.

So I don't currently see any evidence of an unfixed bug.  If, nevertheless, you
believe that a bug is still unfixed, then please provide a reproducer and a fix
patch that clearly explains what it is fixing. 

> There is a null check in post_one_notification for the pipe, most probably
> because it *expects* the pointer to be NULL'd. Also, there is no reason to have
> a dangling pointer stay, it's just a recipe for further bugs.

If you want to send a patch or patches to clean up the code, that is fine, but
please make it super clear what is a cleanup and what is a fix.

- Eric