[PATCH] drm/auth: Only drm_drop_master if it exists

[Jonathan Cavitt <jonathan.cavitt@intel.com>]

It is possible that both dev->master and file_priv->master are NULL when
passed to drm_master_release, which would result in dev being passed to
drm_drop_master (as NULL == NULL here).  This would result in a NULL
pointer dereference when passing dev->master to drm_master_put in
drm_drop_master.

Only call drm_drop_master if dev->master exists.  Also, make sure the
original calling requirement is maintained (dev->master ==
file_priv->master).

This fixes a static analysis issue.

Signed-off-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: David Airlie <airlied@gmail.com>
Cc: Simona Vetter <simona@ffwll.ch>
---
 drivers/gpu/drm/drm_auth.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c
index e17bb0f1f9e0..e5013b870ba0 100644
--- a/drivers/gpu/drm/drm_auth.c
+++ b/drivers/gpu/drm/drm_auth.c
@@ -347,7 +347,7 @@ void drm_master_release(struct drm_file *file_priv)
 	if (!drm_is_current_master_locked(file_priv))
 		goto out;
 
-	if (dev->master == file_priv->master)
+	if (dev->master && dev->master == file_priv->master)
 		drm_drop_master(dev, file_priv);
 out:
 	if (drm_core_check_feature(dev, DRIVER_MODESET) && file_priv->is_master) {
-- 
2.53.0