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Subject: Re: Possible null pointer dereference in adutux.ko
To: Oliver Neukum <oneukum@suse.com>
Cc: johan@kernel.org, gregkh@linuxfoundation.org,
        wsa-dev@sang-engineering.com,
        Alexey Khoroshilov <khoroshilov@ispras.ru>,
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From: Anton Volkov <avolkov@ispras.ru>
Message-ID: <d15c326f-a060-c0e3-b35e-10d02a2c4623@ispras.ru>
Date: Fri, 18 Aug 2017 18:04:59 +0300
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On 15.08.2017 18:58, Oliver Neukum wrote:
> 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
> 

Hello, Oliver.

Looks like deletion of lots of debug print won't solve the race problem 
because there are other places that could potentially try to dereference 
dev->udev when disconnect has already poisoned it. For example in 
adu_open there are calls to usb_fill_int_urb with dev->udev as a 
parameter to be dereferenced inside the function.

There are other possible solutions, if I understand correctly:
1) although it is described that adutux_mutex should be used to protect 
only open_count, it usually protects the whole body of a function, so we 
could probably place it before the locking of dev->mtx;
2) move poisoning of dev->udev after usb_deregister_dev in order to wait 
for all other callbacks to finish.

What do you think?

Regards,
Anton

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