openvswitch: Remove padding from packet before L3+ conntrack processing

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 with a minimum packet length of 64 bytes.

Higher-layer processing functions in netfilter (e.g. nf_ip_checksum(),
and help() in nf_conntrack_ftp) assume skb->len reflects the length of
the L3 header and payload, rather than referring back to
ip_hdr->tot_len or ipv6_hdr->payload_len, and get confused by
lower-layer padding.

In the normal IPv4 receive path, ip_rcv() trims the packet to
ip_hdr->tot_len before invoking netfilter hooks. 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
netfilter hooks.

Currently 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_ip_checksum() fails causing a "nf_ct_tcp: bad TCP checksum" log
message. 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.

In ovs_ct_execute(), trim the skb to the L3 length before higher-layer
processing.

Signed-off-by: Ed Swierk <eswierk@skyportsystems.com>
Acked-by: Pravin B Shelar <pshelar@ovn.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 34 insertions, 0 deletions

diff --git a/net/openvswitch/conntrack.c b/net/openvswitch/conntrack.c
index 62f36cc938ca..c5904f629091 100644
--- a/net/openvswitch/conntrack.c
+++ b/net/openvswitch/conntrack.c
@@ -1098,6 +1098,36 @@ static int ovs_ct_commit(struct net *net, struct sw_flow_key *key,
         return 0;
 }
 
+/* Trim the skb to the length specified by the IP/IPv6 header,
+ * removing any trailing lower-layer padding. This prepares the skb
+ * for higher-layer processing that assumes skb->len excludes padding
+ * (such as nf_ip_checksum). The caller needs to pull the skb to the
+ * network header, and ensure ip_hdr/ipv6_hdr points to valid data.
+ */
+static int ovs_skb_network_trim(struct sk_buff *skb)
+{
+        unsigned int len;
+        int err;
+
+        switch (skb->protocol) {
+        case htons(ETH_P_IP):
+                len = ntohs(ip_hdr(skb)->tot_len);
+                break;
+        case htons(ETH_P_IPV6):
+                len = sizeof(struct ipv6hdr)
+                        + ntohs(ipv6_hdr(skb)->payload_len);
+                break;
+        default:
+                len = skb->len;
+        }
+
+        err = pskb_trim_rcsum(skb, len);
+        if (err)
+                kfree_skb(skb);
+
+        return err;
+}
+
 /* Returns 0 on success, -EINPROGRESS if 'skb' is stolen, or other nonzero
  * value if 'skb' is freed.
  */
@@ -1112,6 +1142,10 @@ int ovs_ct_execute(struct net *net, struct sk_buff *skb,
         nh_ofs = skb_network_offset(skb);
         skb_pull_rcsum(skb, nh_ofs);
 
+        err = ovs_skb_network_trim(skb);
+        if (err)
+                return err;
+
         if (key->ip.frag != OVS_FRAG_TYPE_NONE) {
                 err = handle_fragments(net, key, info->zone.id, skb);
                 if (err)