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

[Ed Swierk <eswierk@skyportsystems.com>]

On Wed, Dec 13, 2017 at 4:58 PM, Pravin Shelar <pshelar@ovn.org> wrote:
> 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?

The nf_ip_checksum() and nf_ip6_checksum() helper functions can easily
be changed to avoid the padding.

My worry is that conntrack is just one of many netfilter hooks that
perform L3+ processing, and may assume that once skb->data points to
the L3 header, skb->len reflects the length of the L3 header and
payload. For example, in nf_conntrack_ftp.c, help() uses skb->len to
determine the length of the FTP payload and the TCP sequence number of
the next packet; this would be thrown off by lower-layer padding.

br_netfilter, a cousin of OVS, has always preserved this
assumption--like ip_rcv() and ip6_rcv(), br_validate_ipv4() and
br_validate_ipv6() trim the skb to the L3 length before they invoke
NF_INET_PRE_ROUTING hooks. Modifying OVS to fit the mold seems more
straightforward than changing this assumption.

>> +       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.

Thanks, I will fix this.

--Ed