[ovs-dev] tc-conntrack: inconsistent behaviour with icmpv6
Ilya Maximets
i.maximets at ovn.org
Wed Mar 10 11:06:52 UTC 2021
Hi, Louis. Thanks for your report!
Marcelo, Paul, could you, please, take a look?
Best regards, Ilya Maximets.
On 3/10/21 8:51 AM, Louis Peens wrote:
> Hi all
>
> We've recently encountered an interesting situation with OVS conntrack
> when offloading to the TC datapath, and would like some feedback. Sorry
> about the longish wall of text, but I'm trying to explain the problem
> as clearly as possible. The very short summary is that there is a mismatch
> in behaviour between the OVS datapath and OVS+TC datapath, and we're
> not sure how to resolve this. Here goes:
>
> We have a set of rules looking like this:
> ovs-ofctl add-flow br0 "table=0,in_port=p1,ct_state=-trk,ipv6,actions=ct(table=1)"
> ovs-ofctl add-flow br0 "table=0,in_port=p2,ct_state=-trk,ipv6,actions=ct(table=1)"
> #post_ct flows"
> ovs-ofctl add-flow br0 "table=1,in_port=p1,ct_state=+trk+new,ipv6,actions=ct(commit),output:p2"
> ovs-ofctl add-flow br0 "table=1,in_port=p2,ct_state=+trk+new,ipv6,actions=ct(commit),output:p1"
> ovs-ofctl add-flow br0 "table=1,in_port=p1,ct_state=+trk+est,ipv6,actions=output:p2"
> ovs-ofctl add-flow br0 "table=1,in_port=p2,ct_state=+trk+est,ipv6,actions=output:p1"
>
> p1/p2 are the endpoints of two different veth pairs, just to keep this simple.
> The rules above work well enough with UDP/TCP traffic, however ICMPv6 packets
> (08:56:39.984375 IP6 2001:db8:0:f101::1 > ff02::1:ff00:2: ICMP6, neighbor solicitation, who has 2001:db8:0:f101::2, length 32)
> breaks this somewhat. With TC offload disabled:
>
> ovs-vsctl --no-wait set Open_vSwitch . other_config:hw-offload=false
>
> we get the following datapath rules:
>
> ovs-appctl dpctl/dump-flows --names
> recirc_id(0x1),in_port(p1),ct_state(-new-est+trk),eth(),eth_type(0x86dd),ipv6(frag=no), packets:2, bytes:172, used:1.329s, actions:drop
> recirc_id(0),in_port(p1),ct_state(-trk),eth(),eth_type(0x86dd),ipv6(frag=no), packets:2, bytes:172, used:1.329s, actions:ct,recirc(0x1)
>
> This part is still fine, we do not have a rule for just matching +trk, so the
> the drop rule is to be expected. The problem however is when we enable TC
> offload:
>
> ovs-vsctl --no-wait set Open_vSwitch . other_config:hw-offload=true
>
> This is the result in the datapath:
>
> ovs-appctl dpctl/dump-flows --names
> ct_state(-trk),recirc_id(0),in_port(p1),eth_type(0x86dd),ipv6(frag=no), packets:2, bytes:144, used:0.920s, actions:ct,recirc(0x1)
> recirc_id(0x1),in_port(p1),ct_state(-new-est-trk),eth(),eth_type(0x86dd),ipv6(frag=no), packets:1, bytes:86, used:0.928s, actions:drop
> recirc_id(0x1),in_port(p1),ct_state(-new-est+trk),eth(),eth_type(0x86dd),ipv6(frag=no), packets:0, bytes:0, used:never, actions:drop
>
> Notice the installation of the two recirc rules, one with -trk and one with +trk,
> with the -trk one being the rule that handles all the next packets. Further
> investigation reveals that something like the following is happening:
>
> 1) The first packet arrives and is handled by the OVS datapath,
> triggering the installation of the two rules like in the non-offloaded
> case. So the recirc_id(0) rule gets installed into tc, and recirc_id(0x1)
> gets installed into the ovs datapath. This bit of code in the OVS module
> makes sure that +trk is set.
>
> /* Update 'key' based on skb->_nfct. If 'post_ct' is true, then OVS has
> * previously sent the packet to conntrack via the ct action.....
> * /
> static void ovs_ct_update_key(const struct sk_buff *skb,
> const struct ovs_conntrack_info *info,
> struct sw_flow_key *key, bool post_ct,
> bool keep_nat_flags)
> {
> ...
> ct = nf_ct_get(skb, &ctinfo);
> if (ct) {//tracked
> ...
> } else if (post_ct) {
> state = OVS_CS_F_TRACKED | OVS_CS_F_INVALID;
> if (info)
> zone = &info->zone;
> }
> __ovs_ct_update_key(key, state, zone, ct);
>
> }
> Obviously this is not the case when the packet was sent to conntrack
> via tc.
>
> 2) The second packet arrives, and now hits the rule installed in
> TC. However, TC does not handle ICMPv6 (Neighbor Solicitation), and explicitely
> clears the tracked bit (net/netfilter/nf_conntrack_proto_icmpv6.c):
>
> int nf_conntrack_icmpv6_error(struct nf_conn *tmpl,
> struct sk_buff *skb,
> unsigned int dataoff,
> const struct nf_hook_state *state)
>
> {
> ...
> type = icmp6h->icmp6_type - 130;
> if (type >= 0 && type < sizeof(noct_valid_new) &&
> noct_valid_new[type]) {
> nf_ct_set(skb, NULL, IP_CT_UNTRACKED);
> return NF_ACCEPT;
> }
> ...
> }
> (The above code gets triggered a few function calls down from act_ct.c)
>
> 3) So now the packet does not hit the +trk rule after the recirc, and leads
> to the installation of the "recirc_id(0x1),..-trk" rule, since +trk wasn't
> set by TC.
>
> This is now the point where we're a bit stuck and is hoping for some ideas
> on how to best resolve this. A workaround is of course just to modify the
> userspace rules to not send the icmp packets to conntrack and that should
> work, but it is a workaround. I think this inconsistency between TC
> offload and non-TC is quite undesirable, and could lead to some interesting
> results, for instance this was first detected by the observation of packets
> getting stuck in a loop in the datapath:
>
> recirc_id(0xe),...ct_state(0/0x20),....,in_port(eth9),eth_type(0x86dd),... ,dp:tc, actions:ct,recirc(0xe)
>
> Where the userspace rule was doing ct to the same table instead of moving to the next table:
>
> ovs-ofctl add-flow br0 "table=0,in_port=eth9,ct_state=-trk,ipv6,actions=ct(table=0)"
>
> So far we've not managed to think of a good way to resolve this in the code.
> I don't think changing the kernel behaviour would be desirable, at least
> not in that specific function as that is common conntrack code. I suspect
> that ideally this is something we can try and address from the OVS side,
> but at this moment I have no idea how this will be achieved, hence this
> email.
>
> Looking forward to get some suggestions on this
>
> Regards
> Louis Peens
>
> PS: Tested on:
> net-next kernel:
> d310ec03a34e Merge tag 'perf-core-2021-02-17' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip
> OVS:
> "cdaa7e0fd dpif-netdev: Fix crash when add dp flow without in_port field."
> +
> "[ovs-dev] [PATCH v3 0/3] Add offload support for ct_state rpl and inv flags"
> (The behaviour before and after the patch series in terms of the problem
> above is the same. Whether the recirc rules end up in the ovs datapath or tc
> datapath doesn't really matter)
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