drm: check for minor master before allowing drop master.

When fast user switching a lot eventually we get to the point,
where we were checking for the wrong thing in this function.

Signed-off-by: Dave Airlie <airlied@redhat.com>

1 files changed, 3 insertions, 0 deletions

diff --git a/drivers/gpu/drm/drm_stub.c b/drivers/gpu/drm/drm_stub.c
index 1b3e0ff57b4..ef878615c49 100644
--- a/drivers/gpu/drm/drm_stub.c
+++ b/drivers/gpu/drm/drm_stub.c
@@ -185,6 +185,9 @@ int drm_dropmaster_ioctl(struct drm_device *dev, void *data,
         if (!file_priv->is_master)
                 return -EINVAL;
 
+        if (!file_priv->minor->master)
+                return -EINVAL;
+
         mutex_lock(&dev->struct_mutex);
         drm_master_put(&file_priv->minor->master);
         file_priv->is_master = 0;