Re: [RFC PATCH 0/2] selinux: avoid nf hooks overhead when not needed

[Paolo Abeni <pabeni@redhat.com>]

On Thu, 2016-04-07 at 14:55 -0400, Paul Moore wrote:
> On Thursday, April 07, 2016 01:45:32 AM Florian Westphal wrote:
> > Paul Moore <paul@paul-moore.com> wrote:
> > > On Wed, Apr 6, 2016 at 6:14 PM, Florian Westphal <fw@strlen.de> wrote:
> > > > netfilter hooks are per namespace -- so there is hook unregister when
> > > > netns is destroyed.
> > > 
> > > Looking around, I see the global and per-namespace registration
> > > functions (nf_register_hook and nf_register_net_hook, respectively),
> > > but I'm looking to see if/how newly created namespace inherit
> > > netfilter hooks from the init network namespace ... if you can create
> > > a network namespace and dodge the SELinux hooks, that isn't a good
> > > thing from a SELinux point of view, although it might be a plus
> > > depending on where you view Paolo's original patches ;)
> > 
> > Heh :-)
> > 
> > If you use nf_register_net_hook, the hook is only registered in the
> > namespace.
> > 
> > If you use nf_register_hook, the hook is put on a global list and
> > registed in all existing namespaces.
> > 
> > New namespaces will have the hook added as well (see
> > netfilter_net_init -> nf_register_hook_list in netfilter/core.c )
> >
> > Since nf_register_hook is used it should be impossible to get a netns
> > that doesn't call these hooks.
> 
> Great, thanks.
>  
> > > > Do you think it makes sense to rework the patch to delay registering
> > > > of the netfiler hooks until the system is in a state where they're
> > > > needed, without the 'unregister' aspect?
> > > 
> > > I would need to see the patch to say for certain, but in principle
> > > that seems perfectly reasonable and I think would satisfy both the
> > > netdev and SELinux camps - good suggestion.  My main goal is to drop
> > > the selinux_nf_ip_init() entirely so it can't be used as a ROP gadget.
> > > 
> > > We might even be able to trim the secmark_active and peerlbl_active
> > > checks in the SELinux netfilter hooks (an earlier attempt at
> > > optimization; contrary to popular belief, I do care about SELinux
> > > performance), although that would mean that enabling the network
> > > access controls would be one way ... I guess you can disregard that
> > > last bit, I'm thinking aloud again.
> > 
> > One way is fine I think.
> 
> Yes, just disregard my second paragraph above.
>  
> > > > Ideally this would even be per netns -- in perfect world we would
> > > > be able to make it so that a new netns are created with an empty
> > > > hook list.
> > > 
> > > In general SELinux doesn't care about namespaces, for reasons that are
> > > sorta beyond the scope of this conversation, so I would like to stick
> > > to a all or nothing approach to enabling the SELinux netfilter hooks
> > > across namespaces.  Perhaps we can revisit this at a later time, but
> > > let's keep it simple right now.
> > 
> > Okay, I'd prefer to stick to your recommendation anyway wrt. to selinux
> > (Casey, I read your comment regarding smack. Noted, we don't want to
> > break smack either...)
> > 
> > I think that in this case the entire question is:
> > 
> > In your experience, how likely is a config where selinux is enabled BUT the
> > hooks are not needed (i.e., where we hit the
> > 
> > if (!selinux_policycap_netpeer)
> >     return NF_ACCEPT;
> > 
> > if (!secmark_active && !peerlbl_active)
> >    return NF_ACCEPT;
> > 
> > tests inside the hooks)?  If such setups are uncommon we should just
> > drop this idea or at least put it on the back burner until the more
> > expensive netfilter hooks (conntrack, cough) are out of the way.
> 
> A few years ago I would have said that it is relatively uncommon for admins to 
> enable the SELinux network access controls; it was typically just 
> government/intelligence agencies who had very strict access control 
> requirements and represented a small portion of SELinux users.  However, over 
> the past few years I've been fielding more and more questions from admins/devs 
> in the virtualization space who are interested in some of these capabilities; 
> it isn't clear to me how many of these people are switching it on, but there 
> is definitely more interest than I have seen in the past and the interested is 
> centered around some rather common use cases.
> 
> So, to summarize, I don't know ;)
> 
> If you've got bigger sources of overhead, my opinion would be to go tackle 
> those first.  Perhaps I can even find the time to work on the 
> SELinux/netfilter stuff while you are off slaying the bigger dragons, no 
> promises at the moment.

Double checking if I got the above correctly.

Will be ok if we post a v2 version of this series, removing the hooks
de-registration bits, but preserving the selinux nf-hooks and
socket_sock_rcv_skb() on-demand/delayed registration ? Will that fit
with the post-init read only memory usage that you are planning ?

Regards,

Paolo