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Date:   Sat, 18 Apr 2020 21:09:44 -0700
From:   Dmitry Torokhov <dmitry.torokhov@gmail.com>
To:     Alan Stern <stern@rowland.harvard.edu>
Cc:     Julian Squires <julian@cipht.net>,
        Hans de Goede <hdegoede@redhat.com>,
        Jiri Kosina <jikos@kernel.org>,
        Benjamin Tissoires <benjamin.tissoires@redhat.com>,
        syzbot <syzbot+7bf5a7b0f0a1f9446f4c@syzkaller.appspotmail.com>,
        linux-input@vger.kernel.org, andreyknvl@google.com,
        gregkh@linuxfoundation.org, ingrassia@epigenesys.com,
        Kernel development list <linux-kernel@vger.kernel.org>,
        USB list <linux-usb@vger.kernel.org>,
        syzkaller-bugs@googlegroups.com
Subject: Re: KASAN: use-after-free Read in usbhid_close (3)
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 <Pine.LNX.4.44L0.2004182158020.26218-100000@netrider.rowland.org>
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Hi Alan,

On Sat, Apr 18, 2020 at 10:16:32PM -0400, Alan Stern wrote:
> linux-input people:
> 
> syzbot has found a bug related to USB/HID/input, and I have narrowed it
> down to the wacom driver.  As far as I can tell, the problem is caused
> the fact that drivers/hid/wacom_sys.c calls input_register_device()
> in several places, but it never calls input_unregister_device().
> 
> I know very little about the input subsystem, but this certainly seems 
> like a bug.

Wacom driver uses devm_input_allocate_device(), so unregister should
happen automatically on device removal once we exit wacom_probe().

> 
> When the device is unplugged, the disconnect pathway doesn't call
> hid_hw_close().  That routine doesn't get called until the user closes
> the device file (which can be long after the device is gone and
> hid_hw_stop() has run).  Then usbhid_close() gets a use-after-free
> error when it tries to access data structures that were deallocated by
> usbhid_stop().  No doubt there are other problems too, but this is
> the one that syzbot found.

Unregistering the input device should result in calling wacom_close()
(if device was previously opened), which, as far as I can tell, calls
hid_hw_close().

I wonder if it is valid to call hid_hw_stop() before hid_hw_close()?

It could be that we again get confused by the "easiness" of devm APIs
and completely screwing up unwind order.

Thanks.

-- 
Dmitry