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

[Pablo Neira Ayuso <pablo@netfilter.org>]

On Thu, Mar 28, 2013 at 03:24:37PM -0400, Stefan Berger wrote:
> On 03/28/2013 03:09 PM, Pablo Neira Ayuso wrote:
> >On Thu, Mar 28, 2013 at 01:55:09PM -0400, Stefan Berger wrote:
> >>On 03/28/2013 01:17 PM, Pablo Neira Ayuso wrote:
> >>>On Thu, Mar 28, 2013 at 10:54:01AM -0400, Stefan Berger wrote:
> >>>>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
> >>>Conntrack is already internally validating that directions are correct
> >>>for you, so no need for those --ctdir. Let me explain why:
> >>>
> >>>If conntrack gets an ICMP echo reply entering through the NEW state,
> >>>it will consider it invalid since it is not coming as reply to an ICMP
> >>>echo request.
> >>[...]
> >>>In sum: The --ctdir is not providing more security. We did not have it
> >>>originally in the `state' match, it was a late extension to the
> >>>conntrack match.
> >>>
> >>>My advice here: Just rely on conntrack states and drop invalid
> >>>traffic, it will do the direction validation that you're trying to
> >>>achieve with that rule-set.
> >>I don't see that removing a filtering rule, as can be done by an
> >>nwfilter user, invalidates the connection tracking state so that a
> >>rule dropping upon INVALID state would then kick in. IMO the
> >>connection is still in ESTABLISHED state and thus will act on a rule
> >>checking on ESTABLISHED state. A simple test here:
> >>
> >>iptables -I INPUT 1 -m state --state INVALID -j DROP
> >>iptables -I INPUT 2 -p icmp -m state --state ESTABLISHED -j ACCEPT
> >>iptables -I INPUT 3 -p icmp -j ACCEPT
> >>
> >>Now ping that machine. Pings should work now
> >>
> >>Following what you said
> >>
> >>iptables -D INPUT -p icmp -j ACCEPT
> >>
> >>should now cause the first rule to kick in for that ICMP stream now
> >>that the rule is gone. This is not the case with my machine and the
> >>ping simply continues -- in this case I have used a RHEL 6
> >>installation with 2.6.32 kernel.
> >If default policy is DROP, then no rules will match, so the ping will
> >be dropped.
> 
> Unless it runs into a rule '-m state --state ESTABLISHED -j ACCEPT',
> which I would say is typical for stateful filtering.

Not sure what you mean. The first packet of an ICMP echo request will
not ever match -m state --state ESTABLISHED.

> The point is '--ctdir' helped us before to cut off traffic and we
> still need it.
> 
> >
> >The rule with the INVALID state only matches if, for example,
> >conntrack sees an ICMP echo reply without having seen an echo request
> >before.
> >
> Well, yeah, but this is not the case we're after.