netfilter: reject: skip csum verification for protocols that don't support it

Some protocols have other means to verify the payload integrity
(AH, ESP, SCTP) while others are incompatible with nf_ip(6)_checksum
implementation because checksum is either optional or might be
partial (UDPLITE, DCCP, GRE). Because nf_ip(6)_checksum was used
to validate the packets, ip(6)tables REJECT rules were not capable
to generate ICMP(v6) errors for the protocols mentioned above.

This commit also fixes the incorrect pseudo-header protocol used
for IPv4 packets that carry other transport protocols than TCP or
UDP (pseudo-header used protocol 0 iso the proper value).

Signed-off-by: Alin Nastac <alin.nastac@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

include/net/netfilter/ipv4/nf_reject.h

@@ -5,6 +5,7 @@
 #include <linux/skbuff.h>
 #include <net/ip.h>
 #include <net/icmp.h>
+#include <net/netfilter/nf_reject.h>
 
 void nf_send_unreach(struct sk_buff *skb_in, int code, int hook);
 void nf_send_reset(struct net *net, struct sk_buff *oldskb, int hook);

include/net/netfilter/ipv6/nf_reject.h

@@ -3,6 +3,7 @@
 #define _IPV6_NF_REJECT_H
 
 #include <linux/icmpv6.h>
+#include <net/netfilter/nf_reject.h>
 
 void nf_send_unreach6(struct net *net, struct sk_buff *skb_in, unsigned char code,
 		      unsigned int hooknum);

include/net/netfilter/nf_reject.h

@@ -0,0 +1,27 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _NF_REJECT_H
+#define _NF_REJECT_H
+
+static inline bool nf_reject_verify_csum(__u8 proto)
+{
+	/* Skip protocols that don't use 16-bit one's complement checksum
+	 * of the entire payload.
+	 */
+	switch (proto) {
+		/* Protocols with other integrity checks. */
+		case IPPROTO_AH:
+		case IPPROTO_ESP:
+		case IPPROTO_SCTP:
+
+		/* Protocols with partial checksums. */
+		case IPPROTO_UDPLITE:
+		case IPPROTO_DCCP:
+
+		/* Protocols with optional checksums. */
+		case IPPROTO_GRE:
+			return false;
+	}
+	return true;
+}
+
+#endif /* _NF_REJECT_H */