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

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Date:   Tue, 15 Aug 2017 15:59:50 +0300
From:   Anton Volkov <avolkov@...ras.ru>
To:     johan@...nel.org, gregkh@...uxfoundation.org,
        wsa-dev@...g-engineering.com
Cc:     linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
        ldv-project@...uxtesting.org,
        Alexey Khoroshilov <khoroshilov@...ras.ru>
Subject: Possible null pointer dereference in adutux.ko

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.

-- Anton Volkov
Linux Verification Center, ISPRAS
web: http://linuxtesting.org
e-mail: avolkov@...ras.ru