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 <oleksandr_andrushchenko@epam.com>
Reported-by: Andrii Chepurnyi <andrii_chepurnyi@epam.com>
Cc: "Noralf Trønnes" <noralf@tronnes.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180522141304.18646-1-andr2000@gmail.com

1 files changed, 7 insertions, 7 deletions

diff --git a/drivers/gpu/drm/drm_drv.c b/drivers/gpu/drm/drm_drv.c
index f6910ebe4d0e..cc2675550e28 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);