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Date:   Tue, 6 Aug 2019 10:19:37 -0400 (EDT)
From:   Alan Stern <stern@rowland.harvard.edu>
To:     Oliver Neukum <oneukum@suse.com>
cc:     Andrey Konovalov <andreyknvl@google.com>,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
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        LKML <linux-kernel@vger.kernel.org>,
        USB list <linux-usb@vger.kernel.org>
Subject: Re: KASAN: use-after-free Read in device_release_driver_internal
In-Reply-To: <1565095011.8136.20.camel@suse.com>
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On Tue, 6 Aug 2019, Oliver Neukum 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.

Only one interface (numbered 234!).

> If you agree. I am attaching a putative fix.

Your patch adds a line saying:

> +	usb_get_intf(acm->control); /* undone in destroy() */

but I don't see any destroy() function in that source file.  Did you 
mean acm_port_destruct()?

In any case, I don't know if this missing "get" would cause the 
problem, but it might well.

Alan Stern