tcp: improve credential handling in syncache

When adding a syncache entry, take a reference count of the
credentials while the inp is still locked.
Thanks to markj@ for providing a hint regarding the root cause.

Reported by:		David Marker
Reviewed by:		glebius
Tested by:		David Marker
Fixes:			cbc9438f05 ("tcp: improve ref count handling when processing SYN")
MFC after:		3 days
Sponsored by:		Netflix, Inc.
Differential Revision:	https://reviews.freebsd.org/D53380

sys/netinet/tcp_syncache.c

@@ -1380,6 +1380,7 @@ syncache_add(struct in_conninfo *inc, struct tcpopt *to, struct tcphdr *th,
 	struct tcpcb *tp;
 	struct socket *rv = NULL;
 	struct syncache *sc = NULL;
+	struct ucred *cred;
 	struct syncache_head *sch;
 	struct mbuf *ipopts = NULL;
 	u_int ltflags;
@@ -1408,6 +1409,7 @@ syncache_add(struct in_conninfo *inc, struct tcpopt *to, struct tcphdr *th,
 	 */
 	KASSERT(SOLISTENING(so), ("%s: %p not listening", __func__, so));
 	tp = sototcpcb(so);
+	cred = V_tcp_syncache.see_other ? NULL : crhold(so->so_cred);
 
 #ifdef INET6
 	if (inc->inc_flags & INC_ISIPV6) {
@@ -1636,16 +1638,16 @@ syncache_add(struct in_conninfo *inc, struct tcpopt *to, struct tcphdr *th,
 	/*
 	 * sc_cred is only used in syncache_pcblist() to list TCP endpoints in
 	 * TCPS_SYN_RECEIVED state when V_tcp_syncache.see_other is false.
-	 * Therefore, store the credentials and take a reference count only
-	 * when needed:
+	 * Therefore, store the credentials only when needed:
 	 * - sc is allocated from the zone and not using the on stack instance.
 	 * - the sysctl variable net.inet.tcp.syncache.see_other is false.
 	 * The reference count is decremented when a zone allocated sc is
 	 * freed in syncache_free().
 	 */
-	if (sc != &scs && !V_tcp_syncache.see_other)
-		sc->sc_cred = crhold(so->so_cred);
-	else
+	if (sc != &scs && !V_tcp_syncache.see_other) {
+		sc->sc_cred = cred;
+		cred = NULL;
+	} else
 		sc->sc_cred = NULL;
 	sc->sc_port = port;
 	sc->sc_ipopts = ipopts;
@@ -1783,6 +1785,8 @@ donenoprobe:
 		tcp_fastopen_decrement_counter(tfo_pending);
 
 tfo_expanded:
+	if (cred != NULL)
+		crfree(cred);
 	if (sc == NULL || sc == &scs) {
 #ifdef MAC
 		mac_syncache_destroy(&maclabel);