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Message-ID: <CAO_EM_mDa17fQx+--RDSyr1_qKSW7ZKxtt+p3J9yt23F8m_F1w@mail.gmail.com>
Date: Thu, 14 Dec 2017 12:05:02 -0800
From: Ed Swierk <eswierk@...portsystems.com>
To: Pravin Shelar <pshelar@....org>
Cc: ovs-dev <ovs-dev@...nvswitch.org>,
Linux Kernel Network Developers <netdev@...r.kernel.org>,
Lance Richardson <lrichard@...hat.com>,
Benjamin Warren <ben@...portsystems.com>,
Keith Holleman <holleman@...portsystems.com>
Subject: Re: [PATCH] openvswitch: Trim off padding before L3 conntrack processing
On Wed, Dec 13, 2017 at 4:58 PM, Pravin Shelar <pshelar@....org> wrote:
> On Tue, Dec 12, 2017 at 8:17 AM, Ed Swierk <eswierk@...portsystems.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@...portsystems.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
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