netfilter: nft_fwd_netdev: check ttl/hl before forwarding

[ Upstream commit 1dfd95b ]

Drop packets if their ttl/hl is too small for forwarding.

Fixes: d32de98 ("netfilter: nft_fwd_netdev: allow to forward packets via neighbour layer")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Sasha Levin <sashal@kernel.org>

Author: Florian Westphal

net/netfilter/nft_fwd_netdev.c

@@ -116,6 +116,11 @@ static void nft_fwd_neigh_eval(const struct nft_expr *expr,
 			goto out;
 		}
 		iph = ip_hdr(skb);
+		if (iph->ttl <= 1) {
+			verdict = NF_DROP;
+			goto out;
+		}
+
 		ip_decrease_ttl(iph);
 		neigh_table = NEIGH_ARP_TABLE;
 		break;
@@ -132,6 +137,11 @@ static void nft_fwd_neigh_eval(const struct nft_expr *expr,
 			goto out;
 		}
 		ip6h = ipv6_hdr(skb);
+		if (ip6h->hop_limit <= 1) {
+			verdict = NF_DROP;
+			goto out;
+		}
+
 		ip6h->hop_limit--;
 		neigh_table = NEIGH_ND_TABLE;
 		break;