[libvirt] [PATCH 4/5] network: regain guest network connectivity after firewalld switch to nftables

[Daniel P. Berrangé]

On Tue, Jan 15, 2019 at 12:43:05PM -0500, Laine Stump wrote:
> On 1/15/19 11:39 AM, Daniel P. Berrangé wrote:
> > On Wed, Jan 09, 2019 at 09:57:36PM -0500, Laine Stump wrote:
> > > From: Laine Stump <laine at redhat.com>
> > > 
> > > In the past (when both libvirt and firewalld used iptables), if either
> > > libvirt's rules *OR* firewalld's rules accepted a packet, it would be
> > > accepted. This was because libvirt and firewalld rules were processed
> > > by the same kernel hook.
> > > 
> > > But now firewalld can use nftables for its backend, while libvirt's
> > > firewall rules are still using iptables; iptables rules are still
> > > processed, but at a different time during packet processing
> > > (i.e. during a different hook) than the firewalld nftables rules. The
> > > result is that a packet must be accepted by *BOTH* the libvirt
> > > iptables rules *AND* the firewalld nftable rules in order to be
> > > accepted.
> > > 
> > > This causes pain because
> > > 
> > > 1) libvirt always adds rules to permit DNS and DHCP (and sometimes
> > > TFTP) from guests to the local host. But libvirt's bridges are in
> > > firewalld's "default" zone (which is usually the zone called
> > > "public"). The public zone allows ssh, but doesn't allow DNS, DHCP, or
> > > TFTP. So even though libvirt's rules allow the DHCP and DNS traffic,
> > > the firewalld rules dont, thus guests connected to libvirt's bridges
> > > can't acquire an IP address from DHCP, nor can they make DNS queries
> > > to the DNS server libvirt has setup on the host.
> > > 
> > > 2) firewalld's higher level "rich rules" don't yet have the ability to
> > > configure the acceptance of forwarded traffic (traffic that is going
> > > somewhere beyond the host), so any traffic that needs to be forwarded
> > > is rejected by the public zone's default "reject" policy (which
> > > rejects all traffic in the zone not specifically allowed by the rules
> > > in the zone, whether that traffic is destined to be forwarded or
> > > locally received by the host).
> > > 
> > > libvirt can't send "direct" nftables rules (firewalld only supports
> > > that for iptables), so we can't solve this problem by just sending
> > > explicit nftables rules instead of explicit iptables rules (which, if
> > > it could be done, would place libvirt's rules in the same hook as
> > > firewalld's native rules, and thus eliminate the need for packets to
> > > be accepted by both libvirt's and firewalld's own rules).
> > > 
> > > However, we can take advantage of a quirk in firewalld zones that have
> > > a default policy of "accept" (meaning any packet that doesn't match a
> > > specific rule in the zone will be *accepted*) - this default accept will
> > > also accept forwarded traffic (not just traffic destined for the host).
> > > 
> > > Putting each network's bridge in a new zone called "libvirt" which has
> > > a default policy of accept will allow the forwarded traffic to pass,
> > > but the same default accept policy that fixes forwarded traffic also
> > > causes *all* traffic from guest to host to be accepted. To solve this
> > > new problem, we can take advantage of a new feature in firewalld
> > > (currently slated for firewalld-0.7.0) - priorities for rich rules -
> > > to add a low priority rule that rejects all local traffic (but leaves
> > > alone all forwarded traffic).
> > Ok, so we depend on newer firewalld. Any older version using nftables
> > will still be broken.
> > 
> > I think it would be quite desirable if we queried the firewalld version
> > and backend and reported a fatal error to the user if they try to
> > start a virtual network when we know it will be broken.
> > 
> > You can get version from /org/fedoraproject/FirewallD1
> > 
> >      org.freedesktop.Properties.Get("org.fedoraproject.FirewallD1", "version")
> > 
> > And backend from /org/fedoraproject/FirewallD1/config
> > 
> >      org.freedesktop.Properties.Get("org.fedoraproject.FirewallD1.config", "FirewallBackend")
> 
> 
> Depending on the firewalld version won't give us correct results, since the
> patches to support rule priorities could be / will be backported to older
> firewalld versions on some distros.
> 
> 
> Of course we could make failure to set the zone to "libvirt" a fatal error -
> that is really the most reliable proxy we have for the question "Does this
> firewalld support rule priorities?" That's a bit draconian though, since it
> means that "new" libvirt will fail to start networks on all distros that
> haven't yet updated their firewalld version (or backported the necessary
> patches). In particular, if they haven't updated firewalld, but they're
> still using the iptables backend anyway, we shouldn't be whining (although
> for consistency we should still set the zone to "libvirt" when possible,
> even if the backend is iptables,)

I feel we should enforce failure to set libvirt zone as a fatal
error *if* we query that FirewallBackend property == nftables.
That is a situation we know will never work correctly, without
opening a security hole in the libvirt rules.

We should also enforce failure if setting zone failed and
version >= 0.7.0, regardless of what backend is set, as we
expect that to always work.

Essentially only case we don't want to report an error is
in version < 0.7.0 and backend is iptables, as that is
safe to use.

This should avoid issues with distros that choose to backport.


> > > +<rule priority='127'>
> > > +  <reject/>
> > > +</rule>
> > > +<service name='dhcp'/>
> > > +<service name='dhcpv6'/>
> > > +<service name='dns'/>
> > > +<service name='ssh'/>
> > > +<service name='tftp'/>
> > > +</zone>
> > I wonder if we should have an configure time check so that we do
> > *not* install this file if built agains an old firewalld version.
> > By unconditionally installing it we're probably causing firewalld
> > to emit an error message in syslog which some sysadmin is almost
> > certainly to report a bug about
> 
> 
> 1) see the comment above about the unreliability of version checks.
> 
> 
> 2) The version / build of firewalld present at build time is really
> irrelevant. As a matter of fact, we currently don't require firewalld to be
> present *at all* at build time (or at runtime - we just use it if it's there
> and active, otherwise we go directly to iptables/ebtables).

If configure installs it by default, but provides a switch, we can
use  --with-firewall-zone / --without-firewalld-zone explicitly in
the RPM specs. We know exactly which Fedora / RHEL versions need
this in so can avoid installing it where we know if isn't wanted.

> 3) I actually *want* the bug reported to distro maintainers, to push them to
> update their firewalld sooner rather than later.

If we install it based on version where nftables supprot was first
introduced in firewalld, rather than when it was fixed, we would
still get useful bug reports without spamming people with errors on
distros where nftables isn't even supported by firewalld.

> So here's an idea - how about if we do this:
> 
> 
> Once when libvirtd is started:
> 
>   retrieve a list of zones and check for presence of the libvirt zone. Save
> this somewhere.
> 
> Each time a network is started:
> 
> if (libvirt zone doesn't exist) {
> 
>     if (firewalldbackend == nftables) {
> 
>        Error "Hey you!! Change the firewalld backend to nftables or update
> firewalld!!"
> 
>     }
> 
> } else {
> 
>     put the bridge in the libvirt zone (regardless of nftables/iptables)
> 
> }

That doesn't solve the issue with installing an invalid zone
file for older firewalld which could generate syslog errors
on firewalld startup.

> (in the meantime, we haven't relied on version number checks, and a system
> with old firewalld can still build a libvirt that supports new firewalld)

As above I don't think we need to rely on version numbers for te compile
time check, as long as we provide the --with/--without flags to let the
distro vendors override it.


Regards,
Daniel
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