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Subject: Re: Possible null pointer dereference in adutux.ko
From: Oliver Neukum <oneukum@suse.com>
To: Anton Volkov <avolkov@ispras.ru>, johan@kernel.org,
        gregkh@linuxfoundation.org, wsa-dev@sang-engineering.com
Cc: Alexey Khoroshilov <khoroshilov@ispras.ru>,
        ldv-project@linuxtesting.org, linux-kernel@vger.kernel.org,
        linux-usb@vger.kernel.org
Date: Tue, 15 Aug 2017 15:20:17 +0200
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References: <fc719c9c-db6a-9254-2d6d-918946c5a18d@ispras.ru>
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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