drm: Do not drop root privileges for a fancier younger process

When a second process opens the device and master transferrence is
complete, we walk the list of open devices and remove their
authentication. This also revokes our root privilege. Instead of simply
dropping the authentication, this patch reverts the authenticated state
back to its original value.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/drm_fops.c

@@ -239,7 +239,8 @@ static int drm_open_helper(struct inode *inode, struct file *filp,
 
 	priv->ioctl_count = 0;
 	/* for compatibility root is always authenticated */
-	priv->authenticated = capable(CAP_SYS_ADMIN);
+	priv->always_authenticated = capable(CAP_SYS_ADMIN);
+	priv->authenticated = priv->always_authenticated;
 	priv->lock_count = 0;
 
 	INIT_LIST_HEAD(&priv->lhead);
@@ -533,7 +534,7 @@ int drm_release(struct inode *inode, struct file *filp)
 		list_for_each_entry(temp, &dev->filelist, lhead) {
 			if ((temp->master == file_priv->master) &&
 			    (temp != file_priv))
-				temp->authenticated = 0;
+				temp->authenticated = temp->always_authenticated;
 		}
 
 		/**

include/drm/drmP.h

@@ -413,6 +413,7 @@ struct drm_prime_file_private {
 
 /** File private data */
 struct drm_file {
+	int always_authenticated;
 	int authenticated;
 	struct pid *pid;
 	kuid_t uid;