[PATCH] openvswitch: Trim off padding before L3 conntrack processing

[Ed Swierk <eswierk@skyportsystems.com>]

A short IPv4 packet may have up to 6 bytes of padding following the IP
payload when received on an Ethernet device.

In the normal IPv4 receive path, ip_rcv() trims the packet to
ip_hdr->tot_len before invoking NF_INET_PRE_ROUTING hooks (including
conntrack). Then any subsequent L3+ processing steps, like
nf_checksum(), use skb->len as the length of the packet, rather than
referring back to ip_hdr->tot_len. In the IPv6 receive path, ip6_rcv()
does the same using ipv6_hdr->payload_len.

In the OVS conntrack receive path, this trimming does not occur, so
the checksum verification in tcp_header() fails, printing "nf_ct_tcp:
bad TCP checksum". Extra zero bytes don't affect the checksum, but the
length in the IP pseudoheader does. That length is based on skb->len,
and without trimming, it doesn't match the length the sender used when
computing the checksum.

With this change, OVS conntrack trims IPv4 and IPv6 packets prior to
L3 processing.

Signed-off-by: Ed Swierk <eswierk@skyportsystems.com>
---
 net/openvswitch/conntrack.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/net/openvswitch/conntrack.c b/net/openvswitch/conntrack.c
index d558e882ca0c..3a7c9215c431 100644
--- a/net/openvswitch/conntrack.c
+++ b/net/openvswitch/conntrack.c
@@ -1105,12 +1105,29 @@ int ovs_ct_execute(struct net *net, struct sk_buff *skb,
 		   const struct ovs_conntrack_info *info)
 {
 	int nh_ofs;
+	unsigned int nh_len;
 	int err;
 
 	/* The conntrack module expects to be working at L3. */
 	nh_ofs = skb_network_offset(skb);
 	skb_pull_rcsum(skb, nh_ofs);
 
+	/* Trim to L3 length since nf_checksum() doesn't expect padding. */
+	switch (skb->protocol) {
+	case htons(ETH_P_IP):
+		nh_len = ntohs(ip_hdr(skb)->tot_len);
+		break;
+	case htons(ETH_P_IPV6):
+		nh_len = ntohs(ipv6_hdr(skb)->payload_len)
+			+ sizeof(struct ipv6hdr);
+		break;
+	default:
+		nh_len = skb->len;
+	}
+	err = pskb_trim_rcsum(skb, nh_len);
+	if (err)
+		return err;
+
 	if (key->ip.frag != OVS_FRAG_TYPE_NONE) {
 		err = handle_fragments(net, key, info->zone.id, skb);
 		if (err)
-- 
2.11.0