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From:   Andrey Konovalov <andreyknvl@google.com>
Date:   Wed, 7 Aug 2019 15:44:43 +0200
Message-ID: <CAAeHK+zDVmxgjkZ6dR-sk1=99-Aj=Z4wwxaRCaOXeuYYG3-bUw@mail.gmail.com>
Subject: Re: KASAN: use-after-free Read in device_release_driver_internal
To:     Oliver Neukum <oneukum@suse.com>,
        syzbot <syzbot+1b2449b7b5dc240d107a@syzkaller.appspotmail.com>
Cc:     syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
        Alan Stern <stern@rowland.harvard.edu>,
        LKML <linux-kernel@vger.kernel.org>,
        USB list <linux-usb@vger.kernel.org>
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On Wed, Aug 7, 2019 at 3:44 PM Andrey Konovalov <andreyknvl@google.com> wrote:
>
> On Wed, Aug 7, 2019 at 3:38 PM Oliver Neukum <oneukum@suse.com> wrote:
> >
> > Am Dienstag, den 06.08.2019, 14:50 +0200 schrieb Andrey Konovalov:
> > > On Tue, Aug 6, 2019 at 2:36 PM Oliver Neukum <oneukum@suse.com> wrote:
> > > >
> > > > Am Donnerstag, den 01.08.2019, 14:47 -0400 schrieb Alan Stern:
> > > > >
> > > > > I think this must be caused by an unbalanced refcount.  That is,
> > > > > something must drop one more reference to the device than it takes.
> > > > > That would explain why the invalid access occurs inside a single
> > > > > bus_remove_device() call, between the klist_del() and
> > > > > device_release_driver().
> > > > >
> > > > > The kernel log indicates that the device was probed by rndis_wlan,
> > > > > rndis_host, and cdc_acm, all of which got errors because of the
> > > > > device's bogus descriptors.  Probably one of them is messing up the
> > > > > refcount.
> > > >
> > > > Hi,
> > > >
> > > > you made me look at cdc-acm. I suspect
> > > >
> > > > cae2bc768d176bfbdad7035bbcc3cdc973eb7984 ("usb: cdc-acm: Decrement tty port's refcount if probe() fail")
> > > >
> > > > is buggy decrementing the refcount on the interface in destroy()
> > > > even before the refcount is increased.
> > > >
> > > > Unfortunately I cannot tell from the bug report how many and which
> > > > interfaces the emulated test device has. Hence it is unclear to me,
> > > > when exactly probe() would fail cdc-acm.
> > > >
> > > > If you agree. I am attaching a putative fix.
> > >
> > > Let's see if it fixes the issue.
> > >
> > > #syz fix: https://github.com/google/kasan.git 6a3599ce
> >
> > Hi,
> >
> > did this ever produce a result? I saw none.
>
> Hm, that's weird, maybe that's caused by putting the bot into CC. Let
> me try that again.
>
> #syz fix: https://github.com/google/kasan.git 6a3599ce

Oh, wait, it should be syz test =)

#syz test: https://github.com/google/kasan.git 6a3599ce

>
> >
> >         Regards
> >                 Oliver
> >
> > --
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