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From:   Dmitry Vyukov <dvyukov@google.com>
Date:   Wed, 11 Dec 2019 16:30:06 +0100
Message-ID: <CACT4Y+ZSmwr4y2VrUxZSvFCL0Ws4cp6T5FwyVRg_CqhCf354HQ@mail.gmail.com>
Subject: Re: KASAN: use-after-free Read in usbvision_v4l2_open
To:     Andrey Konovalov <andreyknvl@google.com>
Cc:     Alan Stern <stern@rowland.harvard.edu>,
        syzbot <syzbot+c7b0ec009a216143df30@syzkaller.appspotmail.com>,
        Hans Verkuil <hverkuil@xs4all.nl>,
        Souptick Joarder <jrdr.linux@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>, linux-media@vger.kernel.org,
        USB list <linux-usb@vger.kernel.org>,
        Mauro Carvalho Chehab <mchehab@kernel.org>,
        Richard Fontana <rfontana@redhat.com>,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
        Thomas Gleixner <tglx@linutronix.de>
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On Wed, Dec 11, 2019 at 3:23 PM Andrey Konovalov <andreyknvl@google.com> wrote:
>
> On Tue, Dec 10, 2019 at 9:13 PM Alan Stern <stern@rowland.harvard.edu> wrote:
> >
> > On Tue, 10 Dec 2019, Andrey Konovalov wrote:
> >
> > > On Tue, Dec 10, 2019 at 8:48 PM Alan Stern <stern@rowland.harvard.edu> wrote:
> >
> > > > This looks like a race in v4l2_open(): The function drops the
> > > > videodev_lock mutex before calling the video driver's open routine, and
> > > > the device can be unregistered during the short time between.
> > > >
> > > > This patch tries to make the race much more likely to happen, for
> > > > testing and verification.
> > > >
> > > > Andrey, will syzbot run the same test with this patch, even though it
> > > > says it doesn't have a reproducer?
> > >
> > > Hi Alan,
> > >
> > > No, unfortunately there's nothing to run if there's no reproducer.
> > > It's technically possible to run the same program log that triggered
> > > the bug initially, but since the bug wasn't reproduced with this log
> > > even without the patch, there isn't much sense in running it with the
> > > patch applied.
> >
> > Actually it does make sense.  That bug was caused by a race, and the
> > patch tries to make the race much more likely to happen, so the same
> > test should fail again.
> >
> > But never mind; I'll try a different approach.  There's another syzbot
> > bug report, one with a reproducer, and with this patch in place it
> > should trigger the same race.  I'll try submitting it that way.
> >
> > By the way, do you know why syzbot sent _two_ reply messages?  One with
> > message ID <00000000000031a0af05995eca0b@google.com> and the other with
> > message ID <000000000000441a4205995eca11@google.com>?  It seems like
> > overkill.
>
> Hm, I'm not sure. Dmitry?

I would assume it received 2 emails (second from syzkaller-bugs@
mailing list) and deduplication logic did not work somehow. So it
replied to both.