netinet6: free in6_ifextra with epoch_call(9)

This is expected to fix the old in6_selecthlim() panics.  The nature of
the panic is that a packet sending thread will obtain the struct ifnet
pointer locklessly and then pick the if_inet6 pointer from it and
dereference it. While the struct ifnet is freed via epoch_call(9), the
struct in6_ifextra until this change was not.  For the forwarded packets,
or locally originated non-TCP packets we were probably safe due to the old
if_dead trick.  But locally originated TCP packets may dereference
in6_ifextra via direct call into in6_selecthlim() from the tcp_output(),
before ip6_output().

NB: hypothetically a similar problem also applies to IPv4's if_inet pointer,
but there are no known panics, yet.

PR:			279653
Reviewed by:		tuexen
Differential Revision:	https://reviews.freebsd.org/D54728

sys/netinet6/in6_ifattach.c

@@ -899,6 +899,19 @@ in6_ifdetach(struct ifnet *ifp)
 	_in6_ifdetach(ifp, 1);
 }
 
+static void
+in6_ifextra_free(epoch_context_t ctx)
+{
+	struct in6_ifextra *ext =
+	     __containerof(ctx, struct in6_ifextra, epoch_ctx);
+
+	COUNTER_ARRAY_FREE(ext->in6_ifstat,
+	    sizeof(struct in6_ifstat) / sizeof(uint64_t));
+	COUNTER_ARRAY_FREE(ext->icmp6_ifstat,
+	    sizeof(struct icmp6_ifstat) / sizeof(uint64_t));
+	free(ext, M_IFADDR);
+}
+
 static void
 in6_ifdeparture(void *arg __unused, struct ifnet *ifp)
 {
@@ -916,14 +929,16 @@ in6_ifdeparture(void *arg __unused, struct ifnet *ifp)
 	if (!VNET_IS_SHUTTING_DOWN(ifp->if_vnet))
 #endif
 		_in6_ifdetach(ifp, 1);
+	/*
+	 * XXXGL: mld and nd bits are left in a consistent state after
+	 * destructors, but I'm not sure if it safe to call lltable_free() here.
+	 * Individual lle entries are epoch(9) protected, but the table itself
+	 * isn't.
+	 */
 	mld_domifdetach(ifp);
 	nd6_ifdetach(ifp);
 	lltable_free(ext->lltable);
-	COUNTER_ARRAY_FREE(ext->in6_ifstat,
-	    sizeof(struct in6_ifstat) / sizeof(uint64_t));
-	COUNTER_ARRAY_FREE(ext->icmp6_ifstat,
-	    sizeof(struct icmp6_ifstat) / sizeof(uint64_t));
-	free(ext, M_IFADDR);
+	NET_EPOCH_CALL(in6_ifextra_free, &ext->epoch_ctx);
 }
 EVENTHANDLER_DEFINE(ifnet_departure_event, in6_ifdeparture, NULL,
     EVENTHANDLER_PRI_ANY);

sys/netinet6/in6_var.h

@@ -531,6 +531,8 @@ struct in6_ifextra {
 	} scope6_id;
 
 	struct lltable *lltable;
+
+	struct epoch_context	epoch_ctx;
 };
 
 #define	LLTABLE6(ifp)	((ifp)->if_inet6->lltable)