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Subject: Re: [PATCH] drm: Fix possible race conditions while unplugging DRM
 device
To:     linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org,
        airlied@linux.ie, daniel.vetter@intel.com, seanpaul@chromium.org,
        gustavo@padovan.org
Cc:     Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>,
        =?UTF-8?Q?Noralf_Tr=c3=b8nnes?= <noralf@tronnes.org>
References: <20180522141304.18646-1-andr2000@gmail.com>
From:   Oleksandr Andrushchenko <andr2000@gmail.com>
Message-ID: <226cb967-2464-767b-c051-95c120f6558a@gmail.com>
Date:   Tue, 29 May 2018 09:25:09 +0300
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On 05/22/2018 05:13 PM, Oleksandr Andrushchenko wrote:
> From: Oleksandr Andrushchenko <oleksandr_andrushchenko@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@epam.com>
> Reported-by: Andrii Chepurnyi <andrii_chepurnyi@epam.com>
> Cc: "Noralf Trønnes" <noralf@tronnes.org>
> ---
>   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);
>