drm: Fix possible race conditions while unplugging DRM device

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 <[email protected]>
Reported-by: Andrii Chepurnyi <[email protected]>
Cc: "Noralf Trønnes" <[email protected]>
Reviewed-by: Daniel Vetter <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

diff --git a/drivers/gpu/drm/drm_drv.c b/drivers/gpu/drm/drm_drv.c
index f6910ebe4d0eda25ad6a78222cf1be2016873278..cc2675550e280a0823acdf4e54c2cbfdfe169534 100644 (file)
--- 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);