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

[Pravin Shelar <pshelar@ovn.org>]

On Thu, Dec 21, 2017 at 7:17 AM, Ed Swierk <eswierk@skyportsystems.com> wrote:
> 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)
This should is unlikely.
> +               kfree_skb(skb);
> +
> +       return err;
> +}
> +
This looks like a generic function, it probably does not belong to OVS
code base.

>  #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;
>
ct-nat is executed within ct action, so I do not see why you you call
skb-trim again from ovs_ct_nat_execute().
ovs_ct_execute() trim should take care of the skb.

>         /* 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
>