linux-kernel - Re: Possible null pointer dereference in adutux.ko

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Date:   Tue, 15 Aug 2017 17:58:55 +0200
From:   Oliver Neukum <oneukum@...e.com>
To:     Anton Volkov <avolkov@...ras.ru>, johan@...nel.org,
        gregkh@...uxfoundation.org, wsa-dev@...g-engineering.com
Cc:     Alexey Khoroshilov <khoroshilov@...ras.ru>,
        ldv-project@...uxtesting.org, linux-kernel@...r.kernel.org,
        linux-usb@...r.kernel.org
Subject: Re: Possible null pointer dereference in adutux.ko

Am Dienstag, den 15.08.2017, 16:38 +0300 schrieb Anton Volkov:
> On 15.08.2017 16:20, Oliver Neukum wrote:
> > 
> > Am Dienstag, den 15.08.2017, 15:59 +0300 schrieb Anton Volkov:
> > > 
> > > Hello.
> > > 
> > > While searching for races in the Linux kernel I've come across
> > > "drivers/usb/misc/adutux.ko" module. Here is a question that I came up
> > > with while analyzing results. Lines are given using the info from Linux
> > > v4.12.
> > > 
> > > Consider the following case:
> > > 
> > > Thread 1:                   Thread 2:
> > > adu_release
> > > ->adu_release_internal      adu_disconnect
> > >       <READ &dev->udev->dev>    dev->udev = NULL
> > >       (adutux.c: line 298)      (adutux.c: line 771)
> > >                                 usb_deregister_dev
> > > 
> > > Comments in the source code point at the possibility of adu_release()
> > > being called separately from adu_disconnect(). adu_release() and
> > > adu_disconnect() acquire different mutexes, so they are not protected
> > > from one another. If adu_disconnect() changes dev->udev before its value
> > > is read in adu_release_internal() there will be a NULL pointer
> > > dereference on a read attempt. Is this case feasible from your point of
> > > view?
> > > 
> > > Thank you for your time.
> > 
> > Hi,
> > 
> > your analysis seems correct to me. In fact it looks like
> > 
> > 66d4bc30d128e7c7ac4cf64aa78cb76e971cec5b
> > USB: adutux: remove custom debug macro
> > 
> > more or less broke disconnect on this driver
> > (the URBs can also finish after dev->udev = NULL)
> > 
> > Do you want to do a fix or do you want me to do it?
> > 
> > 	Regards
> > 		Oliver
> > 
> 
> Hello, Oliver.
> 
> I am not sure about the best way to solve this problem. If you have any 
> ideas about it then it would probably be better if you could handle the 
> fix. Or if you share the ideas I can prepare a patch.

Hi,

given the age of the drivers I would suggest to simply remove the debugging statements

	Regards
		Oliver