linux-kernel - Re: [syzbot] KASAN: use-after-free Read in post_one

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Date:   Tue, 2 Aug 2022 21:09:34 -0700
From:   Eric Biggers <ebiggers@...nel.org>
To:     Siddh Raman Pant <code@...dh.me>
Cc:     syzbot <syzbot+c70d87ac1d001f29a058@...kaller.appspotmail.com>,
        hdanton <hdanton@...a.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>
Subject: Re: [syzbot] KASAN: use-after-free Read in post_one_notification

On Wed, Aug 03, 2022 at 09:34:10AM +0530, Siddh Raman Pant wrote:
> On Wed, 03 Aug 2022 03:57:19 +0530  Eric Biggers <ebiggers@...nel.org> wrote:
> > It appears this was already fixed, so no need for any more activity on this bug:
> > 
> > #syz fix: watchqueue: make sure to serialize 'wqueue->defunct' properly
> > 
> > - Eric
> 
> It doesn't address the dangling pointer remaining in the watch_queue,
> which was the root cause of this crash. The use-after-free happened
> because the pipe was freed but a dangling pointer of it remained in
> a watch_queue, and an attempt to dereference it was there.
> 

I don't think that's true; the pointer doesn't get dereferenced after
watch_queue::defunct is set.  See my message on the other thread where I
explained this: https://lore.kernel.org/lkml/YunKlJCDlmyn2hJ4@sol.localdomain

Of course, if you actually have a reproducer, or a KASAN report, or anything at
all that shows there is still a problem, then please post it.

- Eric