[mutter] wayland/subsurface: Check for circular relationships

[Robert Mader <rmader src gnome org>]

commit 4e9a67acc6e09b012b6034b1928c2cc6ba0cb1bf
Author: Robert Mader <robert mader posteo de>
Date:   Wed Nov 4 19:32:21 2020 +0100

    wayland/subsurface: Check for circular relationships
    
    If a subsurface is equal to or an ancestor of the parent surface
    we currently crash. Check for that case and terminate the client.
    
    Closes https://gitlab.gnome.org/GNOME/mutter/-/issues/1521

 src/wayland/meta-wayland-subsurface.c | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)
---
diff --git a/src/wayland/meta-wayland-subsurface.c b/src/wayland/meta-wayland-subsurface.c
index b4c503d201..bc481db06d 100644
--- a/src/wayland/meta-wayland-subsurface.c
+++ b/src/wayland/meta-wayland-subsurface.c
@@ -541,6 +541,17 @@ surface_handle_parent_surface_destroyed (struct wl_listener *listener,
   surface->sub.parent = NULL;
 }
 
+static gboolean
+is_same_or_ancestor (MetaWaylandSurface *surface,
+                     MetaWaylandSurface *other_surface)
+{
+  if (surface == other_surface)
+    return TRUE;
+  if (other_surface->sub.parent)
+    return is_same_or_ancestor (surface, other_surface->sub.parent);
+  return FALSE;
+}
+
 static void
 wl_subcompositor_get_subsurface (struct wl_client   *client,
                                  struct wl_resource *resource,
@@ -560,6 +571,16 @@ wl_subcompositor_get_subsurface (struct wl_client   *client,
       return;
     }
 
+  if (is_same_or_ancestor (surface, parent))
+    {
+      wl_resource_post_error (resource, WL_SUBCOMPOSITOR_ERROR_BAD_SURFACE,
+                              "Circular relationship between wl_surface@%d "
+                              "and parent surface wl_surface@%d",
+                              wl_resource_get_id (surface->resource),
+                              wl_resource_get_id (parent->resource));
+      return;
+    }
+
   if (!meta_wayland_surface_assign_role (surface,
                                          META_TYPE_WAYLAND_SUBSURFACE,
                                          NULL))