From: Ed Swierk <>

Upstream commit:
    commit 9382fe71c0058465e942a633869629929102843d
    Author: Ed Swierk <>
    Date:   Wed Jan 31 18:48:02 2018 -0800

    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

    Signed-off-by: Ed Swierk <>
    Acked-by: Pravin B Shelar <>
    Signed-off-by: David S. Miller <>

Cc: Ed Swierk <>
Signed-off-by: Greg Rose <>
 datapath/conntrack.c | 34 ++++++++++++++++++++++++++++++++++
 1 file changed, 34 insertions(+)

diff --git a/datapath/conntrack.c b/datapath/conntrack.c
index d58240b..e53b8e3 100644
--- a/datapath/conntrack.c
+++ b/datapath/conntrack.c
@@ -1138,6 +1138,36 @@ 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.
@@ -1152,6 +1182,10 @@ 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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