[PATCH v2] openvswitch: Trim off padding before L3+ netfilter processing

[Ed Swierk <eswierk@skyportsystems.com>]

IPv4 and IPv6 packets may arrive with lower-layer padding that is not
included in the L3 length. For example, 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 netfilter hooks (including
conntrack and nat).

In the IPv6 receive path, ip6_rcv() does the same using
ipv6_hdr->payload_len. Similarly in the br_netfilter receive path,
br_validate_ipv4() and br_validate_ipv6() trim the packet to the L3
length before invoking NF_INET_PRE_ROUTING hooks.

In the OVS conntrack receive path, ovs_ct_execute() pulls the skb to
the L3 header but does not trim it to the L3 length before calling
nf_conntrack_in(NF_INET_PRE_ROUTING). When nf_conntrack_proto_tcp
encounters a packet with lower-layer padding, nf_checksum() fails and
logs "nf_ct_tcp: bad TCP checksum". While extra zero bytes don't
affect the checksum, 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.

The assumption throughout nf_conntrack and nf_nat is that skb->len
reflects the length of the L3 header and payload, so there is no need
to refer back to ip_hdr->tot_len or ipv6_hdr->payload_len.

This change brings OVS into line with other netfilter users, trimming
IPv4 and IPv6 packets prior to L3+ netfilter processing.

Signed-off-by: Ed Swierk <eswierk@skyportsystems.com>
---
v2:
- Trim packet in nat receive path as well as conntrack
- Free skb on error
---
 net/openvswitch/conntrack.c | 34 ++++++++++++++++++++++++++++++++++
 1 file changed, 34 insertions(+)

diff --git a/net/openvswitch/conntrack.c b/net/openvswitch/conntrack.c
index b27c5c6..1bdc78f 100644
--- a/net/openvswitch/conntrack.c
+++ b/net/openvswitch/conntrack.c
@@ -703,6 +703,33 @@ static bool skb_nfct_cached(struct net *net,
 	return ct_executed;
 }
 
+/* Trim the skb to the L3 length. Assumes the skb is already pulled to
+ * the L3 header. The skb is freed on error.
+ */
+static int skb_trim_l3(struct sk_buff *skb)
+{
+	unsigned int nh_len;
+	int err;
+
+	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)
+		kfree_skb(skb);
+
+	return err;
+}
+
 #ifdef CONFIG_NF_NAT_NEEDED
 /* Modelled after nf_nat_ipv[46]_fn().
  * range is only used for new, uninitialized NAT state.
@@ -715,8 +742,12 @@ static int ovs_ct_nat_execute(struct sk_buff *skb, struct nf_conn *ct,
 {
 	int hooknum, nh_off, err = NF_ACCEPT;
 
+	/* The nat module expects to be working at L3. */
 	nh_off = skb_network_offset(skb);
 	skb_pull_rcsum(skb, nh_off);
+	err = skb_trim_l3(skb);
+	if (err)
+		return err;
 
 	/* See HOOK2MANIP(). */
 	if (maniptype == NF_NAT_MANIP_SRC)
@@ -1111,6 +1142,9 @@ int ovs_ct_execute(struct net *net, struct sk_buff *skb,
 	/* The conntrack module expects to be working at L3. */
 	nh_ofs = skb_network_offset(skb);
 	skb_pull_rcsum(skb, nh_ofs);
+	err = skb_trim_l3(skb);
+	if (err)
+		return err;
 
 	if (key->ip.frag != OVS_FRAG_TYPE_NONE) {
 		err = handle_fragments(net, key, info->zone.id, skb);
-- 
1.9.1