virtio-gpu-virgl: correct parent for blob memory region

When `owner` == `mr`, `object_unparent` will crash:

object_unparent(mr) ->
object_property_del_child(mr, mr) ->
object_finalize_child_property(mr, name, mr) ->
object_unref(mr) ->
object_finalize(mr) ->
object_property_del_all(mr) ->
object_finalize_child_property(mr, name, mr) ->
object_unref(mr) ->
fail on g_assert(obj->ref > 0)

However, passing a different `owner` to `memory_region_init` does not
work. `memory_region_ref` has an optimization where it takes a ref
only on the owner. That means when flatviews are created, it does not
take a ref on the region and you can get a UAF from `flatview_destroy`
called from RCU.

The correct fix therefore is to use `NULL` as the name which will set
the `owner` but not the `parent` (which is still NULL). This allows us
to use `memory_region_ref` on itself while not having to rely on unparent
for cleanup.

Signed-off-by: Joelle van Dyne <j@getutm.app>
Reviewed-by: Akihiko Odaki <odaki@rsg.ci.i.u-tokyo.ac.jp>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Message-Id: <20260103214400.71694-1-j@getutm.app>

hw/display/virtio-gpu-virgl.c

@@ -120,7 +120,7 @@ virtio_gpu_virgl_map_resource_blob(VirtIOGPU *g,
     vmr->g = g;
 
     mr = &vmr->mr;
-    memory_region_init_ram_ptr(mr, OBJECT(mr), "blob", size, data);
+    memory_region_init_ram_ptr(mr, OBJECT(mr), NULL, size, data);
     memory_region_add_subregion(&b->hostmem, offset, mr);
     memory_region_set_enabled(mr, true);
 
@@ -186,7 +186,7 @@ virtio_gpu_virgl_unmap_resource_blob(VirtIOGPU *g,
         /* memory region owns self res->mr object and frees it by itself */
         memory_region_set_enabled(mr, false);
         memory_region_del_subregion(&b->hostmem, mr);
-        object_unparent(OBJECT(mr));
+        object_unref(OBJECT(mr));
     }
 
     return 0;