[ldv-project] A potential race

Fri Jul 8 16:06:49 MSK 2016

Hi!
We have no hardware to test possible fixes. If somebody has it and 
agrees to check our patches, we will prepare them.
Best regards,
Pavel
01.07.2016 20:17, Hans Verkuil пишет:
> On 07/01/2016 05:02 PM, Pavel Andrianov wrote:
>> 01.07.2016 19:53, Hans Verkuil пишет:
>>> On 07/01/2016 04:39 PM, Pavel Andrianov wrote:
>>>> Hi!
>>>>>>>> There is a potential race condition between usbvision_v4l2_close and usbvision_disconnect. The possible scenario may be the following. usbvision_disconnect starts execution, assigns usbvision->remove_pending = 1, and is interrupted
>>>> (rescheduled) after mutex_unlock. After that usbvision_v4l2_close is executed, decrease usbvision->user-- , checks usbvision->remove_pending, executes usbvision_release and finishes. Then usbvision_disconnect continues its execution. It checks
>>>> usbversion->user (it is already 0) and also execute usbvision_release. Thus, release is executed twice. The same situation may
>>>> occur if usbvision_v4l2_close is interrupted by usbvision_disconnect. Moreover, the same problem is in usbvision_radio_close. In all these cases the check before call usbvision_release under mutex_lock protection does not solve the problem, because there may occur an open() after the check and the race takes place again. The question is: why the usbvision_release
>>>> is called from close() (usbvision_v4l2_close and usbvision_radio_close)? Usually release functions are called from
>>>> disconnect.
>>> Please don't use html mail, mailinglists will silently reject this.
>>>>>> The usbvision driver is old and unloved and known to be very bad code. It needs a huge amount of work to make all this work correctly.
>>>>>> I don't see anyone picking this up...
>>>>>> Regards,
>>>>>> 	Hans
>> If you know the driver, could you, please, explain me, why
>> usbvision_release is called from close functions (usbvision_v4l2_close
>> and usbvision_radio_close) and not only from disconnect? Thanks!
>>> Because the author didn't know what he was doing. Although, to be fair, we have much better
> solutions for this. But who is willing to put in the time to fix this properly?
>> The basic idea was: if someone still has a video/radio node open when disconnect happens, then
> we leave it to the last close to call release, otherwise we can call release right away.
>> It needs to be rewritten.
>> If you're volunteering to clean this up, then I can give pointers.
>> Regards,
>> 	Hans

-- 
Pavel Andrianov
Linux Verification Center, ISPRAS
web: http://linuxtesting.org
e-mail: andrianov at ispras.ru