netfilter: conntrack: remove error callback and handle icmp from core

icmp(v6) are the only two layer four protocols that need the error()
callback (to handle icmp errors that are related to an established
connections, e.g. packet too big, port unreachable and the like).

Remove the error callback and handle these two special cases from the core.

Signed-off-by: Florian Westphal <[e-mail]>
Signed-off-by: Pablo Neira Ayuso <[e-mail]>

include/net/netfilter/nf_conntrack_l4proto.h

···
 	/* Called when a conntrack entry is destroyed */
 	void (*destroy)(struct nf_conn *ct);
 
-	int (*error)(struct nf_conn *tmpl, struct sk_buff *skb,
-		     unsigned int dataoff,
-		     const struct nf_hook_state *state);
-
 	/* called by gc worker if table is full */
 	bool (*can_early_drop)(const struct nf_conn *ct);
 
···
 	struct module *me;
 };
 
+int nf_conntrack_icmpv4_error(struct nf_conn *tmpl,
+			      struct sk_buff *skb,
+			      unsigned int dataoff,
+			      const struct nf_hook_state *state);
+
+int nf_conntrack_icmpv6_error(struct nf_conn *tmpl,
+			      struct sk_buff *skb,
+			      unsigned int dataoff,
+			      const struct nf_hook_state *state);
 /* Existing built-in generic protocol */
 extern const struct nf_conntrack_l4proto nf_conntrack_l4proto_generic;
 

net/netfilter/nf_conntrack_core.c

···
 	return 0;
 }
 
+/*
+ * icmp packets need special treatment to handle error messages that are
+ * related to a connection.
+ *
+ * Callers need to check if skb has a conntrack assigned when this
+ * helper returns; in such case skb belongs to an already known connection.
+ */
+static unsigned int __cold
+nf_conntrack_handle_icmp(struct nf_conn *tmpl,
+			 struct sk_buff *skb,
+			 unsigned int dataoff,
+			 u8 protonum,
+			 const struct nf_hook_state *state)
+{
+	int ret;
+
+	if (state->pf == NFPROTO_IPV4 && protonum == IPPROTO_ICMP)
+		ret = nf_conntrack_icmpv4_error(tmpl, skb, dataoff, state);
+#if IS_ENABLED(CONFIG_IPV6)
+	else if (state->pf == NFPROTO_IPV6 && protonum == IPPROTO_ICMPV6)
+		ret = nf_conntrack_icmpv6_error(tmpl, skb, dataoff, state);
+#endif
+	else
+		return NF_ACCEPT;
+
+	if (ret <= 0) {
+		NF_CT_STAT_INC_ATOMIC(state->net, error);
+		NF_CT_STAT_INC_ATOMIC(state->net, invalid);
+	}
+
+	return ret;
+}
+
 unsigned int
 nf_conntrack_in(struct sk_buff *skb, const struct nf_hook_state *state)
 {
···
 
 	l4proto = __nf_ct_l4proto_find(state->pf, protonum);
 
-	/* It may be an special packet, error, unclean...
-	 * inverse of the return code tells to the netfilter
-	 * core what to do with the packet. */
-	if (l4proto->error != NULL) {
-		ret = l4proto->error(tmpl, skb, dataoff, state);
+	if (protonum == IPPROTO_ICMP || protonum == IPPROTO_ICMPV6) {
+		ret = nf_conntrack_handle_icmp(tmpl, skb, dataoff,
+					       protonum, state);
 		if (ret <= 0) {
-			NF_CT_STAT_INC_ATOMIC(state->net, error);
-			NF_CT_STAT_INC_ATOMIC(state->net, invalid);
 			ret = -ret;
 			goto out;
 		}

net/netfilter/nf_conntrack_proto_icmp.c

···
 }
 
 /* Small and modified version of icmp_rcv */
-static int
-icmp_error(struct nf_conn *tmpl,
-	   struct sk_buff *skb, unsigned int dataoff,
-	   const struct nf_hook_state *state)
+int nf_conntrack_icmpv4_error(struct nf_conn *tmpl,
+			      struct sk_buff *skb, unsigned int dataoff,
+			      const struct nf_hook_state *state)
 {
 	const struct icmphdr *icmph;
 	struct icmphdr _ih;
···
 	.pkt_to_tuple		= icmp_pkt_to_tuple,
 	.invert_tuple		= icmp_invert_tuple,
 	.packet			= icmp_packet,
-	.error			= icmp_error,
 	.destroy		= NULL,
 	.me			= NULL,
 #if IS_ENABLED(CONFIG_NF_CT_NETLINK)

net/netfilter/nf_conntrack_proto_icmpv6.c

···
 			       IPPROTO_ICMPV6, "%s", msg);
 }
 
-static int
-icmpv6_error(struct nf_conn *tmpl,
-	     struct sk_buff *skb,
-	     unsigned int dataoff,
-	     const struct nf_hook_state *state)
+int nf_conntrack_icmpv6_error(struct nf_conn *tmpl,
+			      struct sk_buff *skb,
+			      unsigned int dataoff,
+			      const struct nf_hook_state *state)
 {
 	const struct icmp6hdr *icmp6h;
 	struct icmp6hdr _ih;
···
 	.pkt_to_tuple		= icmpv6_pkt_to_tuple,
 	.invert_tuple		= icmpv6_invert_tuple,
 	.packet			= icmpv6_packet,
-	.error			= icmpv6_error,
 #if IS_ENABLED(CONFIG_NF_CT_NETLINK)
 	.tuple_to_nlattr	= icmpv6_tuple_to_nlattr,
 	.nlattr_tuple_size	= icmpv6_nlattr_tuple_size,