linux-kernel - Re: KASAN: use-after-free Read in device_release_driver

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Date:   Tue, 6 Aug 2019 14:50:50 +0200
From:   Andrey Konovalov <andreyknvl@...gle.com>
To:     Oliver Neukum <oneukum@...e.com>
Cc:     Alan Stern <stern@...land.harvard.edu>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        syzbot <syzbot+1b2449b7b5dc240d107a@...kaller.appspotmail.com>,
        LKML <linux-kernel@...r.kernel.org>,
        USB list <linux-usb@...r.kernel.org>
Subject: Re: KASAN: use-after-free Read in device_release_driver_internal

On Tue, Aug 6, 2019 at 2:36 PM Oliver Neukum <oneukum@...e.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

>
>         Regards
>                 Oliver

View attachment "0001-usb-cdc-acm-make-sure-a-refcount-is-taken-early-enou.patch" of type "text/x-patch" (1751 bytes)