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

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.

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.