ipv4: Handle attempt to delete multipath route when fib_info contains an nh reference

Gwangun Jung reported a slab-out-of-bounds access in fib_nh_match:
    fib_nh_match+0xf98/0x1130 linux-6.0-rc7/net/ipv4/fib_semantics.c:961
    fib_table_delete+0x5f3/0xa40 linux-6.0-rc7/net/ipv4/fib_trie.c:1753
    inet_rtm_delroute+0x2b3/0x380 linux-6.0-rc7/net/ipv4/fib_frontend.c:874

Separate nexthop objects are mutually exclusive with the legacy
multipath spec. Fix fib_nh_match to return if the config for the
to be deleted route contains a multipath spec while the fib_info
is using a nexthop object.

Fixes: 493ced1a ("ipv4: Allow routes to use nexthop objects")
Fixes: 6bf92d70 ("net: ipv4: fix route with nexthop object delete warning")
Reported-by: NGwangun Jung <exsociety@gmail.com>
Signed-off-by: NDavid Ahern <dsahern@kernel.org>
Reviewed-by: NIdo Schimmel <idosch@nvidia.com>
Tested-by: NIdo Schimmel <idosch@nvidia.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/fib_semantics.c

@@ -888,13 +888,13 @@ int fib_nh_match(struct net *net, struct fib_config *cfg, struct fib_info *fi,
 		return 1;
 	}
 
+	/* cannot match on nexthop object attributes */
+	if (fi->nh)
+		return 1;
+
 	if (cfg->fc_oif || cfg->fc_gw_family) {
 		struct fib_nh *nh;
 
-		/* cannot match on nexthop object attributes */
-		if (fi->nh)
-			return 1;
-
 		nh = fib_info_nh(fi, 0);
 		if (cfg->fc_encap) {
 			if (fib_encap_match(net, cfg->fc_encap_type,

tools/testing/selftests/net/fib_nexthops.sh

@@ -1223,6 +1223,11 @@ ipv4_fcnal()
 	log_test $rc 0 "Delete nexthop route warning"
 	run_cmd "$IP route delete 172.16.101.1/32 nhid 12"
 	run_cmd "$IP nexthop del id 12"
+
+	run_cmd "$IP nexthop add id 21 via 172.16.1.6 dev veth1"
+	run_cmd "$IP ro add 172.16.101.0/24 nhid 21"
+	run_cmd "$IP ro del 172.16.101.0/24 nexthop via 172.16.1.7 dev veth1 nexthop via 172.16.1.8 dev veth1"
+	log_test $? 2 "Delete multipath route with only nh id based entry"
 }
 
 ipv4_grp_fcnal()