netfilter: nfnetlink_queue: unbreak SCTP traffic

when packet is enqueued with nfqueue and GSO is enabled, checksum
calculation has to take into account the protocol, as SCTP uses a
32 bits CRC checksum.

Enter skb_gso_segment() path in case of SCTP GSO packets because
skb_zerocopy() does not support for GSO_BY_FRAGS.

Joint work with Pablo.
Signed-off-by: Antonio Ojea <aojea@google.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/core/dev.c

@@ -3386,6 +3386,7 @@ int skb_crc32c_csum_help(struct sk_buff *skb)
 out:
 	return ret;
 }
+EXPORT_SYMBOL(skb_crc32c_csum_help);
 
 __be16 skb_network_protocol(struct sk_buff *skb, int *depth)
 {

net/netfilter/nfnetlink_queue.c

@@ -540,6 +540,14 @@ static int nfqnl_put_bridge(struct nf_queue_entry *entry, struct sk_buff *skb)
 	return -1;
 }
 
+static int nf_queue_checksum_help(struct sk_buff *entskb)
+{
+	if (skb_csum_is_sctp(entskb))
+		return skb_crc32c_csum_help(entskb);
+
+	return skb_checksum_help(entskb);
+}
+
 static struct sk_buff *
 nfqnl_build_packet_message(struct net *net, struct nfqnl_instance *queue,
 			   struct nf_queue_entry *entry,
@@ -602,7 +610,7 @@ nfqnl_build_packet_message(struct net *net, struct nfqnl_instance *queue,
 	case NFQNL_COPY_PACKET:
 		if (!(queue->flags & NFQA_CFG_F_GSO) &&
 		    entskb->ip_summed == CHECKSUM_PARTIAL &&
-		    skb_checksum_help(entskb))
+		    nf_queue_checksum_help(entskb))
 			return NULL;
 
 		data_len = READ_ONCE(queue->copy_range);
@@ -1014,7 +1022,7 @@ nfqnl_enqueue_packet(struct nf_queue_entry *entry, unsigned int queuenum)
 		break;
 	}
 
-	if ((queue->flags & NFQA_CFG_F_GSO) || !skb_is_gso(skb))
+	if (!skb_is_gso(skb) || ((queue->flags & NFQA_CFG_F_GSO) && !skb_is_gso_sctp(skb)))
 		return __nfqnl_enqueue_packet(net, queue, entry);
 
 	nf_bridge_adjust_skb_data(skb);