[PATCH] drm: Fix possible race conditions while unplugging DRM device

[Oleksandr Andrushchenko]

On 05/29/2018 10:49 AM, Daniel Vetter wrote:
> On Tue, May 29, 2018 at 10:09:57AM +0300, Oleksandr Andrushchenko wrote:
>> On 05/29/2018 10:02 AM, Daniel Vetter wrote:
>>> On Tue, May 22, 2018 at 05:13:04PM +0300, Oleksandr Andrushchenko wrote:
>>>> From: Oleksandr Andrushchenko <oleksandr_andrushchenko at epam.com>
>>>>
>>>> When unplugging a hotpluggable DRM device we first unregister it
>>>> with drm_dev_unregister and then set drm_device.unplugged flag which
>>>> is used to mark device critical sections with drm_dev_enter()/
>>>> drm_dev_exit() preventing access to device resources that are not
>>>> available after the device is gone.
>>>> But drm_dev_unregister may lead to hotplug uevent(s) fired to
>>>> user-space on card and/or connector removal, thus making it possible
>>>> for user-space to try accessing a disconnected device.
>>>>
>>>> Fix this by first making sure device is properly marked as
>>>> disconnected and only then unregister it.
>>>>
>>>> Fixes: bee330f3d672 ("drm: Use srcu to protect drm_device.unplugged")
>>>>
>>>> Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko at epam.com>
>>>> Reported-by: Andrii Chepurnyi <andrii_chepurnyi at epam.com>
>>>> Cc: "Noralf Trønnes" <noralf at tronnes.org>
>>> Nice catch.
>>>
>>> Reviewed-by: Daniel Vetter <daniel.vetter at ffwll.ch>
>>>
>>> I think you need to push this to drm-misc-next-fixes to make sure it's on
>>> the 4.17 train.
>> Sure, after I have r-b from Noralf
> Noralf's occasionally occupied with other things and doesn't have time to
> look at patches. I think it's ok to just push after a few more days, even
> if he doesn't respond. Same holds for other people really.
> -Daniel
>
Applied to drm-misc-next-fixes
>>> -Daniel
>>>
>>>> ---
>>>>    drivers/gpu/drm/drm_drv.c | 14 +++++++-------
>>>>    1 file changed, 7 insertions(+), 7 deletions(-)
>>>>
>>>> diff --git a/drivers/gpu/drm/drm_drv.c b/drivers/gpu/drm/drm_drv.c
>>>> index b553a6f2ff0e..7af748ed1c58 100644
>>>> --- a/drivers/gpu/drm/drm_drv.c
>>>> +++ b/drivers/gpu/drm/drm_drv.c
>>>> @@ -369,13 +369,6 @@ EXPORT_SYMBOL(drm_dev_exit);
>>>>     */
>>>>    void drm_dev_unplug(struct drm_device *dev)
>>>>    {
>>>> -	drm_dev_unregister(dev);
>>>> -
>>>> -	mutex_lock(&drm_global_mutex);
>>>> -	if (dev->open_count == 0)
>>>> -		drm_dev_put(dev);
>>>> -	mutex_unlock(&drm_global_mutex);
>>>> -
>>>>    	/*
>>>>    	 * After synchronizing any critical read section is guaranteed to see
>>>>    	 * the new value of ->unplugged, and any critical section which might
>>>> @@ -384,6 +377,13 @@ void drm_dev_unplug(struct drm_device *dev)
>>>>    	 */
>>>>    	dev->unplugged = true;
>>>>    	synchronize_srcu(&drm_unplug_srcu);
>>>> +
>>>> +	drm_dev_unregister(dev);
>>>> +
>>>> +	mutex_lock(&drm_global_mutex);
>>>> +	if (dev->open_count == 0)
>>>> +		drm_dev_put(dev);
>>>> +	mutex_unlock(&drm_global_mutex);
>>>>    }
>>>>    EXPORT_SYMBOL(drm_dev_unplug);
>>>> -- 
>>>> 2.17.0
>>>>
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