fix use-after-free: require new fsm deferred dealloc, check for term

API doc: require osmo_fsm_set_dealloc_ctx().

mgcp_client during delete: do not reparent the FSM when it is already
terminating.

I have recently discovered a vulnerability: if an endpoint FSM deallocates
during event handling of a successful MGCP response, this causes a
use-after-free; and once that is fixed, a state change on the already
terminated FSM causes a pointer corruption by using already cleaned data
structures. osmo_fsm_set_dealloc_ctx() fixes the use-after-free, and
osmo_fsm_set_term_stops_actions() fixes the pointer corruption.

Related: Ib7fce7b7d54dfb87af97544796680919e5929a50 (osmo-bsc),
         I08c03946605aa12e0a5ce8b3c773704ef5327a7a (osmo-msc)
Depends: Ief4dba9ea587c9b4aea69993e965fbb20fb80e78 (libosmocore),
         I0adc13a1a998e953b6c850efa2761350dd07e03a (libosmocore)
Change-Id: I7df2e9202b04e7ca7366bb0a8ec53cf3bb14faf3

src/libosmo-mgcp-client/mgcp_client_endpoint_fsm.c

@@ -255,6 +255,9 @@ struct osmo_mgcpc_ep *osmo_mgcpc_ep_fi_mgwep(struct osmo_fsm_inst *fi)
  * MGCP messages to set up the endpoint will be sent on the given mgcp_client, as soon as the first
  * osmo_mgcpc_ep_ci_request() is invoked.
  *
+ * IMPORTANT: To avoid use-after-free problems, using this FSM requires use of deferred FSM deallocation using
+ * osmo_fsm_set_dealloc_ctx(), e.g. using osmo_select_main_ctx(OTC_SELECT) with osmo_select_main_ctx() as main loop.
+ *
  * A typical sequence of events would be:
  *
  *    ep = osmo_mgcpc_ep_alloc(..., mgcp_client_rtpbridge_wildcard(client));

src/libosmo-mgcp-client/mgcp_client_fsm.c

@@ -708,6 +708,9 @@ void mgcp_conn_delete(struct osmo_fsm_inst *fi)
 
 	OSMO_ASSERT(mgcp_ctx);
 
+	if (fi->proc.terminating)
+		return;
+
 	/* Unlink FSM from parent */
 	osmo_fsm_inst_unlink_parent(fi, NULL);