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

Kyle Moffett
2007-10-05T05:50:16Z

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.

>>> 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.

On the other hand, the "user" and "role" fields in SELinux are
already fairly flexible. The one constant is that user=>role and
role=>type must both be allowed for a label to be valid. Other than
that, the constraints specify what must be true for a transition to
be allowed. For example the standard strict reference policy
includes this bit:

constrain process transition (
(u1 == u2) or
(t1 == can_change_process_identity and t2 == process_user_target) or
(t1 == cron_source_domain and (t2 == cron_job_domain or u2 ==
system_u)) or
(t1 == can_system_change and u2 == system_u) or
(t1 == process_uncond_exempt)
);

Basically all constraints on a particular access vector must be
satisfied for that to be allowed (in addition to other things). For
the above example, a process running exec() may only change its user
if one of the following:

* It's a login-like program and is starting a user entrypoint
* It's a cron-like program and is starting a user or system cronjob
* It's a process allowed to start system processes (admin runs
initscripts)
* It's unconditionally exempt

By creating types and assigning meaningful attributes to those types
you may restrict the changing of the "user" and "role" however you
would like, including not at all.

Really SELinux is just a fairly elaborate security state-machine.
Each process on the system is in a given "state", defined by its
label, and "state transitions" are only allowed based on the rules
defined in the database. Since there are 4 extremely common security
models that people like to combine there are 4 fields in each SELinux
label:
(A) User: User A may not poke user B's processes/data, even if
they happen to be running in the same UNIX UID
(B) Role: Users may only perform operations for the role they are
currently logged in as
(C) Type-enforcement: An apache process may only read files that
it needs to operate correctly
(D) Multi-level: Top-secret data can't magically become
unclassified

Perhaps the thing to do would be to make it possible to compile out
the portions which people don't want. That would certainly satisfy
Casey, he would build only the "Type-enforcement" portion or only the
"Multi-level" portion and be able to do exactly the same things he
does now with a bit finer granularity of operations. By defining the
lists of operations he cares about for "r" (read), "w" (write),
"x" (exec), "a" (append), and requires-capabilities, you can just
give those permissions directly in a simplified SELinux-type policy.

>>> I'm not yet annoyed enough to go implement an iptables like
>>> interface to the LSM enhancing it with more generic mechanism to
>>> make the problem simpler, but I'm getting there. Perhaps next
>>> time I'm bored.
>>
>> I think a fair amount of what we need is already done in SELinux,
>> and efforts would be better spent in figuring out what seems too
>> complicated in SELinux and making it simpler. Probably a fair
>> amount of that just means better tools.
>
> How about thinking of it another way.
>
> Perform the split up you talked about above and move the table
> matching into the LSM hooks.
>
> Use something like the iptables action and match to module mapping
> code so we can have multiple modules compiled in and useable at the
> same time with the LSM hooks.
>
> I think it is firmly established that selling SElinux to everyone
> is politically untenable. However enhancing the LSM (even if it is
> mostly selinux code movement down a layer) I think can be sold.
>
> If I could run Serge's isolation code and selinux rules at the same
> time that would be interesting.
>
> My impression is that selinux is one monolithic blob that doesn't
> allow me to incrementally add matching or action features that I
> find interesting.

Well the major problem here is the performance one. As anybody who
has tried to do a thousand-linear-rule IPtables set will quickly tell
you, it makes performance go all to hell. Good iptables frontends
like shorewall build little subchains to keep the fast-path as
efficient as possible but regardless the search is a linear rule-
traversal (IE: O(N) where N is the number of rules you had to test).
In order to get good enough performance out of SELinux they had to
*start* with an O(log(N) rule traversal which means that execution-
order-style rules are impossible, so you *MUST* limit the fields and
prevent any overlap between rules. For example, in SELinux nobody
writes "deny" rules because the interpretation-order is undefined.
Instead you only write "allow" rules for the things you actually want
to allow. The one exception is "neverallow" rules which are verified
at compile time as sanity checks and never actually loaded in the
kernel.

Cheers,
Kyle Moffett