ipv6: fib6_select_path can not use out path for nexthop objects

Brian reported a crash in IPv6 code when using rpfilter with a setup
running FRR and external nexthop objects. The root cause of the crash
is fib6_select_path setting fib6_nh in the result to NULL because of
an improper check for nexthop objects.

More specifically, rpfilter invokes ip6_route_lookup with flowi6_oif
set causing fib6_select_path to be called with have_oif_match set.
fib6_select_path has early check on have_oif_match and jumps to the
out label which presumes a builtin fib6_nh. This path is invalid for
nexthop objects; for external nexthops fib6_select_path needs to just
return if the fib6_nh has already been set in the result otherwise it
returns after the call to nexthop_path_fib6_result. Update the check
on have_oif_match to not bail on external nexthops.

Update selftests for this problem.

Fixes: f88d8ea6 ("ipv6: Plumb support for nexthop object in a fib6_info")
Reported-by: Brian Rak <brak@choopa.com>
Signed-off-by: David Ahern <dsahern@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/route.c

@@ -431,9 +431,12 @@ void fib6_select_path(const struct net *net, struct fib6_result *res,
 	struct fib6_info *sibling, *next_sibling;
 	struct fib6_info *match = res->f6i;
 
-	if ((!match->fib6_nsiblings && !match->nh) || have_oif_match)
+	if (!match->nh && (!match->fib6_nsiblings || have_oif_match))
 		goto out;
 
+	if (match->nh && have_oif_match && res->nh)
+		return;
+
 	/* We might have already computed the hash for ICMPv6 errors. In such
 	 * case it will always be non-zero. Otherwise now is the time to do it.
 	 */

tools/testing/selftests/net/fib_nexthops.sh

@@ -747,6 +747,19 @@ ipv6_fcnal_runtime()
 	run_cmd "$IP nexthop add id 86 via 2001:db8:91::2 dev veth1"
 	run_cmd "$IP ro add 2001:db8:101::1/128 nhid 81"
 
+	# rpfilter and default route
+	$IP nexthop flush >/dev/null 2>&1
+	run_cmd "ip netns exec me ip6tables -t mangle -I PREROUTING 1 -m rpfilter --invert -j DROP"
+	run_cmd "$IP nexthop add id 91 via 2001:db8:91::2 dev veth1"
+	run_cmd "$IP nexthop add id 92 via 2001:db8:92::2 dev veth3"
+	run_cmd "$IP nexthop add id 93 group 91/92"
+	run_cmd "$IP -6 ro add default nhid 91"
+	run_cmd "ip netns exec me ping -c1 -w1 2001:db8:101::1"
+	log_test $? 0 "Nexthop with default route and rpfilter"
+	run_cmd "$IP -6 ro replace default nhid 93"
+	run_cmd "ip netns exec me ping -c1 -w1 2001:db8:101::1"
+	log_test $? 0 "Nexthop with multipath default route and rpfilter"
+
 	# TO-DO:
 	# existing route with old nexthop; append route with new nexthop
 	# existing route with old nexthop; replace route with new