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Message-ID: <1565095011.8136.20.camel@suse.com>
Subject: Re: KASAN: use-after-free Read in device_release_driver_internal
From:   Oliver Neukum <oneukum@suse.com>
To:     Alan Stern <stern@rowland.harvard.edu>,
        Andrey Konovalov <andreyknvl@google.com>
Cc:     syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
        syzbot <syzbot+1b2449b7b5dc240d107a@syzkaller.appspotmail.com>,
        LKML <linux-kernel@vger.kernel.org>,
        USB list <linux-usb@vger.kernel.org>
Date:   Tue, 06 Aug 2019 14:36:51 +0200
In-Reply-To: <Pine.LNX.4.44L0.1908011359580.1305-100000@iolanthe.rowland.org>
References: <Pine.LNX.4.44L0.1908011359580.1305-100000@iolanthe.rowland.org>
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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.

	Regards
		Oliver

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