From 426e41f500f7e8f73141723345828ea1f6da4748 Mon Sep 17 00:00:00 2001
From: Vlad Zahorodnii <vlad.zahorodnii@kde.org>
Date: Wed, 4 Nov 2020 23:34:10 +0200
Subject: [PATCH] Check for subsurface loops

If subsurfaces form a loop, kwin will get stuck in an infinite
recursion while it constructs a window pixmap tree.
---
 src/wayland/subcompositor_interface.cpp | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/src/wayland/subcompositor_interface.cpp b/src/wayland/subcompositor_interface.cpp
index 8fa05ad8fb..0f6c9dec89 100644
--- a/src/wayland/subcompositor_interface.cpp
+++ b/src/wayland/subcompositor_interface.cpp
@@ -50,6 +50,19 @@ void SubCompositorInterfacePrivate::subcompositor_get_subsurface(Resource *resou
         return;
     }
 
+    if (surface == parent) {
+        wl_resource_post_error(resource->handle, error_bad_surface,
+                               "wl_surface@%d cannot be its own parent",
+                               wl_resource_get_id(surface_resource));
+        return;
+    }
+    if (parent->subSurface() && parent->subSurface()->mainSurface() == surface) {
+        wl_resource_post_error(resource->handle, error_bad_surface,
+                               "wl_surface@%d is an ancestor of parent",
+                               wl_resource_get_id(surface_resource));
+        return;
+    }
+
     wl_resource *subsurfaceResource = wl_resource_create(resource->client(), &wl_subsurface_interface,
                                                          resource->version(), id);
     if (!subsurfaceResource) {