Re: [PATCH] Version 3 (2.6.23-rc8) Smack: Simplified Mandatory Access Control Kernel

Serge E. Hallyn
2007-10-08T16:18:55Z

Quoting Casey Schaufler (casey@schaufler-ca.com):
>
> --- Kyle Moffett <mrmacman_g4@mac.com> wrote:
>
> > On Oct 05, 2007, at 00:45:17, Eric W. Biederman wrote:
> > > Kyle Moffett <mrmacman_g4@mac.com> writes:
> > >
> > >> On Oct 04, 2007, at 21:44:02, Eric W. Biederman wrote:
> > >>> SElinux is not all encompassing or it is generally
> > >>> incomprehensible I don't know which. Or someone long ago would
> > >>> have said a better way to implement containers was with a
> > >>> selinux ruleset, here is a selinux ruleset that does that.
> > >>> Although it is completely possible to implement all of the
> > >>> isolation with the existing LSM hooks as Serge showed.
> > >>
> > >> The difference between SELinux and containers is that SELinux (and
> > >> LSM as a whole) returns -EPERM to operations outside the scope of
> > >> the subject, whereas containers return -ENOENT (because it's not
> > >> even in the same namespace).
> > >
> > > Yes. However if you look at what the first implementations were.
> > > Especially something like linux-vserver. All they provided was
> > > isolation. So perhaps you would not see every process ps but they
> > > all had unique pid values.
> > >
> > > I'm pretty certain Serge at least prototyped a simplified version
> > > of that using the LSM hooks. Is there something I'm not remember
> > > in those hooks that allows hiding of information like processes?
> > >
> > > Yes. Currently with containers we are taking that one step farther
> > > as that solves a wider set of problems.
> >
> > IMHO, containers have a subtly different purpose from LSM even though
> > both are about information hiding. Basically a container is
> > information hiding primarily for administrative reasons; either as a
> > convenience to help prevent errors or as a way of describing
> > administrative boundaries. For example, even in an environment where
> > all sysadmins are trusted employees, a few head-honcho sysadmins
> > would get root container access, and all others would get access to
> > specific containers as a way of preventing "oops" errors. Basically
> > a container is about "full access inside this box and no access
> > outside".
> >
> > By contrast, LSM is more strictly about providing *limited* access to
> > resources. For an accounting business all client records would
> > grouped and associated together, however those which have passed this
> > year's review are read-only except by specific staff and others may
> > have information restricted to some subset of the employees.
> >
> > So containers are exclusive subsets of "the system" while LSM should
> > be about non-exclusive information restriction.
>
> Yes. Isolation is a much simpler problem than access control.
>
> > >>> We also have in the kernel another parallel security mechanism
> > >>> (for what is generally a different class of operations) that has
> > >>> been quite successful, and different groups get along quite
> > >>> well, and ordinary mortals can understand it. The linux
> > >>> firewalling code.
> > >>
> > >> Well, I wouldn't go so far as the "ordinary mortals can understand
> > >> it" part; it's still pretty high on the obtuse-o-meter.
> > >
> > > True. Probably a more accurate statement is:`unix command line
> > > power users can and do handle it after reading the docs. That's
> > > not quite ordinary mortals but it feels like it some days. It
> > > might all be perception...
> >
> > I have seen more *wrong* iptables firewalls than I've seen correct
> > ones. Securing TCP/IP traffic properly requires either a lot of
> > training/experience or a good out-of-the-box system like Shorewall
> > which structures the necessary restrictions for you based on an
> > abstract description of the desired functionality. For instance what
> > percentage of admins do you think could correctly set up their
> > netfilter firewalls to log christmas-tree packets, smurfs, etc
> > without the help of some external tool? Hell, I don't trust myself
> > to reliably do it without a lot of reading of docs and testing, and
> > I've been doing netfilter firewalls for a while.
> >
> > The bottom line is that with iptables it is *CRITICAL* to have a good
> > set of interface tools to take the users' "My system is set up
> > like..." description in some form and turn it into the necessary set
> > of efficient security rules. The *exact* same issue applies to
> > SELinux, with 2 major additional problems:
> >
> > 1) Half the tools are still somewhat beta-ish and under heavy
> > development. Furthermore the semi-official reference policy is
> > nowhere near comprehensive and pretty ugly to read (go back to the
> > point about the tools being beta-ish).
> >
> > 2) If you break your system description or translation tools then
> > instead of just your network dying your entire *system* dies.
> >
> >
> > >>> The linux firewalling codes has hooks all throughout the
> > >>> networking stack, just like the LSM has hooks all throughout the
> > >>> rest of linux kernel. There is a difference however. The linux
> > >>> firewalling code in addition to hooks has tables behind those
> > >>> hooks that it consults. There is generic code to walk those
> > >>> tables and consult with different kernel modules to decide if we
> > >>> should drop a packet. Each of those kernel modules provides a
> > >>> different capability that can be used to generate a firewall.
> > >>
> > >> This is almost *EXACTLY* what SELinux provides as an LSM module.
> > >> The one difference is that with SELinux some compromises and
> > >> restrictions have been made so that (theoretically) the resulting
> > >> policy can be exhaustively analyzed to *prove* what it allows and
> > >> disallows. It may be that SELinux should be split into 2 parts,
> > >> one that provides the underlying table-matching and the other
> > >> that uses it to provide the provability guarantees. Here's a
> > >> direct comparison:
> > >>
> > >> netfilter:
> > >> (A) Each packet has src, dst, port, etc that can be matched
> > >> (B) Table of rules applied sequentially (MATCH => ACTION)
> > >> (C) Rules may alter the properties of packets as they are routed/
> > >> bridged/etc
> > >>
> > >> selinux:
> > >> (A) Each object has user, role, and type that can be matched
> > >> (B) Table of rules searched by object parameters (MATCH => allow/
> > >> auditallow/transition)
> > >> (C) Rules may alter the properties of objects through transition
> > >> rules.
> > >
> > > Ok. There is something here.
> > >
> > > However in a generic setup, at least role would be an extended
> > > match criteria provided by the selinux module. It would not be a
> > > core attribute. It would need to depend on some extra
> > > functionality being compiled in.
> >
> > Now see I think *THAT* is where Casey should be going with his SMACK
> > code. Don't add another LSM, start looking at SELinux and figuring
> > out what parts he does not need and how they can be parameterized out
> > at build time for smaller systems.
>
> Good suggestion. In fact, that is exactly how I approached my
> first two attempts at the problem. What you get if you take that
> route is an imposing infrastructure that has virually nothing
> to do and that adds no value to the solution. Programming to the
> LSM interface, on the other hand, allowed me to drastically reduce
> the size and complexity of the implementation.

(tongue-in-cheek)

No no, everyone knows you don't build simpler things on top of more
complicated ones, you go the other way around. So what he was
suggesting was that selinux be re-written on top of smack.

:)

-serge