netfilter: conntrack: mark UDP zero checksum as CHECKSUM_UNNECESSARY

The udp_error function verifies the checksum of incoming UDP packets if
one is set. This has the desirable side effect of setting skb->ip_summed
to CHECKSUM_COMPLETE, signalling that this verification need not be
repeated further up the stack.

Conversely, when the UDP checksum is empty, which is perfectly legal (at least
inside IPv4), udp_error previously left no trace that the checksum had been
deemed acceptable.

This was a problem in particular for nf_reject_ipv4, which verifies the
checksum in nf_send_unreach() before sending ICMP_DEST_UNREACH. It makes
no accommodation for zero UDP checksums unless they are already marked
as CHECKSUM_UNNECESSARY.

This commit ensures packets with empty UDP checksum are marked as
CHECKSUM_UNNECESSARY, which is explicitly recommended in skbuff.h.

Signed-off-by: Kevin Mitchell <kevmitch@arista.com>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_conntrack_proto_udp.c

@@ -63,8 +63,10 @@ static bool udp_error(struct sk_buff *skb,
 	}
 
 	/* Packet with no checksum */
-	if (!hdr->check)
+	if (!hdr->check) {
+		skb->ip_summed = CHECKSUM_UNNECESSARY;
 		return false;
+	}
 
 	/* Checksum invalid? Ignore.
 	 * We skip checking packets on the outgoing path