dbus: gate the bus socket like INIT_SOCKET

The D-Bus socket was bound world read/write, on the reasoning that
SO_PEERCRED authorizes each method anyway.  That leaves the read-only
surface open to every local user, and it quietly ignores --with-group:
a system that restricts initctl to the wheel group still handed the
same service state to anyone who asked over the bus.

Bind it 0660 and chown it to the configured group, the same gate the
fallback socket has always had.  libink takes the mode as an argument
rather than assuming one, since who may connect is the embedder's
policy, not the library's.

The mode is applied at bind(), so there is no window where the socket
is more permissive than intended.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

src/dbus.c

@@ -1341,11 +1341,17 @@ int dbus_init(uev_ctx_t *ctx)
 {
 	dbg("Setting up D-Bus listening socket at %s ...", FINIT_BUS_SOCKET);
 
-	if (link_server_new(&server, FINIT_BUS_SOCKET) < 0) {
+	/* Same access policy as INIT_SOCKET: the bus reaches every
+	 * service operation initctl does, so --with-group has to gate
+	 * both or it gates neither. */
+	if (link_server_new(&server, FINIT_BUS_SOCKET, 0660) < 0) {
 		err(1, "Failed binding D-Bus socket %s", FINIT_BUS_SOCKET);
 		return 1;
 	}
 
+	if (chown(FINIT_BUS_SOCKET, geteuid(), getgroup(DEFGROUP)))
+		err(1, "Failed setting group %s on %s", DEFGROUP, FINIT_BUS_SOCKET);
+
 	if (link_server_add_object(server, "/org/finit/manager",
 				  &manager_vtable, NULL) < 0) {
 		err(1, "Failed registering Manager1 object");