user32: Protect from setting as a window owner one of its successors.

dlls/user32/tests/win.c

diff --git a/dlls/user32/tests/win.c b/dlls/user32/tests/win.c
index 632e382..3af8d75 100644
--- a/dlls/user32/tests/win.c
+++ b/dlls/user32/tests/win.c
@@ -380,6 +380,15 @@
     check_parents( test, desktop, 0, desktop, 0, test, desktop );
 
     /* window is now child of desktop so GWLP_HWNDPARENT changes owner from now on */
+    if (!is_win9x)
+    {
+        ret = (HWND)SetWindowLongPtrA( test, GWLP_HWNDPARENT, (LONG_PTR)test );
+        ok( ret == 0, "GWL_HWNDPARENT return value %p expected 0\n", ret );
+        check_parents( test, desktop, 0, desktop, 0, test, desktop );
+    }
+    else
+        win_skip("Test creates circular window tree under Win9x/WinMe\n" );
+
     ret = (HWND)SetWindowLongPtrA( test, GWLP_HWNDPARENT, (LONG_PTR)child );
     ok( ret == 0, "GWL_HWNDPARENT return value %p expected 0\n", ret );
     check_parents( test, desktop, child, desktop, child, test, desktop );

server/window.c

diff --git a/server/window.c b/server/window.c
index 3b912b0..5fdfd13 100644
--- a/server/window.c
+++ b/server/window.c
@@ -1834,7 +1834,7 @@
 DECL_HANDLER(set_window_owner)
 {
     struct window *win = get_window( req->handle );
-    struct window *owner = NULL;
+    struct window *owner = NULL, *ptr;
 
     if (!win) return;
     if (req->owner && !(owner = get_window( req->owner ))) return;
@@ -1843,6 +1843,17 @@
         set_error( STATUS_ACCESS_DENIED );
         return;
     }
+
+    /* make sure owner is not a successor of window */
+    for (ptr = owner; ptr; ptr = ptr->owner ? get_window( ptr->owner ) : NULL)
+    {
+        if (ptr == win)
+        {
+            set_error( STATUS_INVALID_PARAMETER );
+            return;
+        }
+    }
+
     reply->prev_owner = win->owner;
     reply->full_owner = win->owner = owner ? owner->handle : 0;
 }