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Re: [libvirt] [PATCH v3] nwfilter: probe for inverted ctdir



On 03/28/2013 10:36 AM, Laine Stump wrote:
For reference of people new to this thread, here is the start of the thread:

   https://www.redhat.com/archives/libvir-list/2013-March/msg01403.html

This concerns changes to libvirt to cope with the newly discovered (by
us :-) difference in interpretation of ctdir by different versions of
netfilter.

On 03/28/2013 07:11 AM, Stefan Berger wrote:
On 03/27/2013 09:09 PM, Stefan Berger wrote:
On 03/27/2013 02:01 PM, Eric Blake wrote:
On 03/27/2013 10:30 AM, Laine Stump wrote:
My opinion is that the patch we should apply should be a simple patch
that just removes use of --ctdir. According to the netfilter developer
who responded to the thread on libvirt-users, it doesn't add any extra
security not already provided by conntrack:

https://www.redhat.com/archives/libvirt-users/2013-March/msg00121.html
https://www.redhat.com/archives/libvirt-users/2013-March/msg00128.html

Not being an expert on netfilter internals, I can't dispute his claim.

Does anyone else have an opinion?
What filters specifically caused the use of --ctdir, and are they
broken
if we omit the use of --ctdir?
It depends on how you write the filters that the --ctdir is being used.

iirc: The effect of the --ctdir usage is that if one has an incoming
rule and and outgoing rule with the same IP address on the 'other'
side the check for an ESTABLISHED state is not enough to ACCEPT the
traffic, if one was to remove one of the rules while communication in
both directions was occurring and an immediate cut of the traffic in
one way was expected. The effect so far was that if the rule for the
incoming rule was removed it would cut the incoming traffic
immediately while the traffic in outgoing direction was
uninterrupted. I think that if we remove this now the traffic in both
directions will continue. I will verify tomorrow.
Verified. I have a ping running from the VM to destination 'A' and
from 'A' to the VM. The --ctdir enforces the direction of the traffic
and if one of the following rules is removed, the ping is immediately
cut.

   <rule action='accept' direction='out' priority='500'>
     <icmp/>
   </rule>
   <rule action='accept' direction='in' priority='500'>
     <icmp/>
   </rule>

The ping is not cut anymore upon removal of one of the above rules if
--ctdir was to be removed entirely.
Okay, as I understand from your description, the difference is that when
a ping in one direction is already in action, and you remove the rule
allowing that ping, that existing ping "session" will continue to be
allowed *if* there is still a rule allowing pings in the other
direction. Is that correct? I'm guessing that *new* attempts to ping in
that direction will no longer be allowed though, is that also correct?

For the benefit of Pablo and the other netfilter developers, can you
paste the iptables commands that are generated for the two rules above?
Possibly they can suggest alternative rules that have the desired effect.


First off, there are multiple ways one can write the filtering rules in nwfilter, either stateless or stateful:

http://libvirt.org/formatnwfilter.html#nwfwrite


Thus the filter here is only one example how one can write a stateful filter for traffic from/to a VM:

<filter name='ctdirtest' chain='ipv4' priority='-700'>
<uuid>582c2fe6-569a-f366-58fb-f995f1a559ce</uuid>
  <rule action='accept' direction='out' priority='500'>
    <icmp/>
  </rule>
  <rule action='accept' direction='in' priority='500'>
    <icmp/>
  </rule>
  <rule action='drop' direction='inout' priority='500'>
    <all/>
  </rule>
</filter>

The filter above creates the following types of rules -- some rules are omitted that goto into these user-defined rules.

Chain FI-vnet0 (1 references)
pkts bytes target prot opt in out source destination 6 504 RETURN icmp -- * * 0.0.0.0/0 0.0.0.0/0 state NEW,ESTABLISHED ctdir ORIGINAL 0 0 RETURN icmp -- * * 0.0.0.0/0 0.0.0.0/0 state ESTABLISHED ctdir REPLY
    0     0 DROP       all  --  *      * 0.0.0.0/0            0.0.0.0/0

Chain FO-vnet0 (1 references)
pkts bytes target prot opt in out source destination 0 0 ACCEPT icmp -- * * 0.0.0.0/0 0.0.0.0/0 state ESTABLISHED ctdir REPLY 0 0 ACCEPT icmp -- * * 0.0.0.0/0 0.0.0.0/0 state NEW,ESTABLISHED ctdir ORIGINAL
   53  3235 DROP       all  --  *      * 0.0.0.0/0            0.0.0.0/0

Chain HI-vnet0 (1 references)
pkts bytes target prot opt in out source destination 0 0 RETURN icmp -- * * 0.0.0.0/0 0.0.0.0/0 state NEW,ESTABLISHED ctdir ORIGINAL 0 0 RETURN icmp -- * * 0.0.0.0/0 0.0.0.0/0 state ESTABLISHED ctdir REPLY
    0     0 DROP       all  --  *      * 0.0.0.0/0            0.0.0.0/0


Those may not be the most efficient rules, yet they provide the possibility to remove a rule from the XML and cut the connection immediately due to the ctdir usage.


If they have no alternatives, then I do now agree that we shouldn't just
scrap --ctdir. Then we have two choices:

1) take your patch, which hopefully will successfully guess the polarity
of --ctdir correctly in all cases.

Why 'guess' -- it 'probes' for it and does actual testing.


2) switch unconditionally to the new "correct" polarity of --ctdir,
release-note the heck out of it, and require that any distro with a
kernel old enough to have the old style --ctdir backport at least the
one patch to netfilter to change that.

Not a good idea IMHO because it would break on older kernels.


My comments about (1): while I'll again say that the patch truly is
poetic in its ability to overcome obstacles, it's really a workaround
for a bug, and as time goes on will become less and less relevant (and
more and more difficult to explain/rationalize). I really think I would
prefer to be broken on very old distros rather than have that patch in
the tree (although others may have a different opinion, and I would
gracefully withdraw my objection in that case).

About (2): Fedora at least as far back as F16 has a new enough
kernel/iptables (iptables-1.4.12) that it uses the new polarity for
--ctdir, and no older Fedora is supported. RHEL6 and CentOS6 still have
iptables-1.4.7, so they have the old polarity. I'd be willing to bet
that RHEL6 would take the netfilter patch to change --ctdir; of course
that would leave us with temporary brokenness for anybody who upgraded
their kernel without upgrading libvirt, or vice-versa.

You not only need to convert the filtering rules (and only do it once) but also re-program the users minds that may be used to that odd mixup of directions.


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