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Date:   Sun, 19 Apr 2020 10:07:34 -0400 (EDT)
From:   Alan Stern <stern@rowland.harvard.edu>
To:     Dmitry Torokhov <dmitry.torokhov@gmail.com>
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>, Ping Cheng <pingc@wacom.com>,
        <pinglinux@gmail.com>, <killertofu@gmail.com>
Subject: Re: KASAN: use-after-free Read in usbhid_close (3)
In-Reply-To: <20200419041344.GC166864@dtor-ws>
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On Sat, 18 Apr 2020, Dmitry Torokhov wrote:

> On Sat, Apr 18, 2020 at 09:09:44PM -0700, Dmitry Torokhov wrote:
> > 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()?

No, it isn't.  If it were, for example, why would evdev_disconnect() -> 
evdev_cleanup() need to call input_close_device()?  And why would 
usbhid_disconnect() deallocate the usbhid structure which usbhid_stop()
accesses?

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

That's probably what happened.

Alan Stern

> Let's also add Ping and Jason to the conversation...