From: Ed Swierk <>

[ Upstream commit 9382fe71c0058465e942a633869629929102843d ]

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

Signed-off-by: Ed Swierk <>
Acked-by: Pravin B Shelar <>
Signed-off-by: David S. Miller <>
Signed-off-by: Sasha Levin <>
 net/openvswitch/conntrack.c | 34 ++++++++++++++++++++++++++++++++++
 1 file changed, 34 insertions(+)

diff --git a/net/openvswitch/conntrack.c b/net/openvswitch/conntrack.c
index d558e882ca0c..285f8797c26a 100644
--- a/net/openvswitch/conntrack.c
+++ b/net/openvswitch/conntrack.c
@@ -1097,6 +1097,36 @@ 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.
@@ -1111,6 +1141,10 @@ 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->, skb);
                if (err)

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