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

[Pravin Shelar <pshelar@ovn.org>]

On Tue, Dec 12, 2017 at 8:17 AM, Ed Swierk <eswierk@skyportsystems.com> wrote:
> 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. */
Can you explore if nf_checksum can be changed to avoid the 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;
> +
In case of error skb needs to be freed.