core: harden cgroups-agent forwarding

On dbus1, we receive systemd1.Agent signals via the private socket, hence
it's trusted. However, on kdbus we receive it on the system bus. We must
make sure it's sent by UID=0, otherwise unprivileged users can fake it.

Furthermore, never forward broadcasts we sent ourself. This might happen
on kdbus, as we forward the message on the same bus we received it on,
thus ending up in an endless loop.

src/core/dbus.c

@@ -69,13 +69,37 @@ int bus_send_queued_message(Manager *m) {
 }
 
 static int signal_agent_released(sd_bus_message *message, void *userdata, sd_bus_error *error) {
+        _cleanup_bus_creds_unref_ sd_bus_creds *creds = NULL;
+        const char *cgroup, *me;
         Manager *m = userdata;
-        const char *cgroup;
+        uid_t sender_uid;
+        sd_bus *bus;
         int r;
 
         assert(message);
         assert(m);
 
+        /* ignore recursive events sent by us on the system/user bus */
+        bus = sd_bus_message_get_bus(message);
+        if (!sd_bus_is_server(bus)) {
+                r = sd_bus_get_unique_name(bus, &me);
+                if (r < 0)
+                        return r;
+
+                if (streq_ptr(sd_bus_message_get_sender(message), me))
+                        return 0;
+        }
+
+        /* only accept org.freedesktop.systemd1.Agent from UID=0 */
+        r = sd_bus_query_sender_creds(message, SD_BUS_CREDS_EUID, &creds);
+        if (r < 0)
+                return r;
+
+        r = sd_bus_creds_get_euid(creds, &sender_uid);
+        if (r < 0 || sender_uid != 0)
+                return 0;
+
+        /* parse 'cgroup-empty' notification */
         r = sd_bus_message_read(message, "s", &cgroup);
         if (r < 0) {
                 bus_log_parse_error(r);