[cap-talk] - Bellizzomi - Capabilities and Shapiro's focus, Coyotos, etc.

Valerio Bellizzomi devbox at selnet.org
Thu Nov 30 18:38:59 CST 2006


On 30/11/2006, at 1.27, Jonathan S. Shapiro wrote:

>The tacit assertion that Shapiro *has* a focus must be viewed as an
>unvalidated hypothesis. :-)
>
>On Tue, 2006-11-28 at 16:44 -0800, Jed at Webstart wrote:
>> At 03:38 PM 11/28/2006, Valerio Bellizzomi wrote:
>> >On 28/11/2006, at 13.49, Jed at Webstart wrote:
>> ><snip>
>> > >At this point I personally think any such efforts <to block wall
>banging>
>> > >are likely a waste of time and effort.  I believe that the only way
>to deal
>> > >with wall banging (which effort is in itself likely not a good use
of
>time)
>> > >is to limit shared resources.  I think the 
>> > trade-offs are clear in this area.
>
>I agree with most of this, though limiting shared resources isn't nearly
>enough, because logically unshared resources are often multiplexed onto
>shared resources (consider network connections) and the multiplexing
>points become communication nexi for covert communications.
>
>Having said that, my views about covert channels are predicated on the
>assumption that the bandwidth of covert channels is unimportant. This is
>an unvalidated assumption, and validating it becomes increasingly
>important as defensible systems (of whatever form) become closer to
>reality.
>
>There is no work in the open literature (and no work of any kind that I
>know about) on the engineering of covert channels. That is: no work
>doing any serious investigation of how to construct them, what
>techniques are best for data exfiltration, how to evaluate them, and/or
>characterizing the achievable bandwidth.
>
>I have often thought that it would be useful from a science perspective
>for someone to undertake to build a covert channel SDK and lay down the
>quality evaluation criteria for covert channel developers. I suspect
>that channels of O(100 Kbit/s) are not difficult to construct on current
>systems. Having a well-characterized statement of exposure might alter
>our outlook on whether this issue is important.
>
>As far as I can tell, the only thing stopping the active development of
>covert exfiltration technology is that other techniques are still
>easier. In particular, penetration is required regardless and overt
>channels are too easy to build.
>
>> >I believe limiting shared resource is one of the objectives of Shap's
>> >research (read the "Debunking Linus's Latest" and "From EROS to
>> >Coyotos/BitC: Open Source meets Open Proofs" papers).
>
>This is true, though not because of any concern about covert channels.
>One of the strengths of the KeyKOS design was that all metadata was
>explicitly reified. Mapping structures, for example, are crafted from
>Nodes. Nodes are named by capabilities, and the purchase of Nodes is an
>accounted activity (via the space bank).
>
>This is in marked contrast to every other system I know about, where the
>storage costs of metadata are assumed to be bundled up in the storage
>costs of the data they map. The argument is usually that the mapping

Has this to do with placing the cost of resource allocation on the client
and not on the server ?

>tables occupy logx(p) space, where 'p' is number of pages and 'x' is
>imprecisely known but small; under this assumption one may argue that
>the storage cost can be accounted for as a tax on the storage of the
>pages themselves and requires no independent tracking. The problem with
>this approach is that it ignores the issue of sharing, the issue of free
>riders, the issue of extended durability through reference counts, and
>the fact that in small proceses logx(p) >= p. This is true because the
>formula is incorrect. It should be more correctly stated as
>
>   ceil(logx(p)) + ceil(logy(ceil(logx(p)))) + ...
>
>where each level of the hierarchy has its own log function and cannot
>occupy fractional storage.
>
>By making metadata explicit, KeyKOS forces programmers to address all of
>these issues -- most notably the durability problem. I have often said
>that this is a feature, because when a problem can't be solved it is
>better for an architecture to force the problem to be dealt with than to
>bury it under a rug. KeyKOS did not create the problem; it merely makes
>it impossible to ignore.
>
>But in practice this is quite awkward, because of the durability issue.
>There are no good idioms for negotiating shared durability without a
>bidirectional channel, and this defeats isolation in places where one
>would like to preserve it.
>
>Note that the same issue exists in MLS systems as a covert channel in
>the observability of quota availability. Ironically, the reference count
>and implicit observability is a narrower channel than any explicitly
>negotiated durability.
>
>The other issue is that scheduling of domains with multiple clients is a
>nightmare that nobody knows how to cleanly solve. Priority inversion is
>not robust in message passing systems unless message flow observes a
>strict hierarchy. There are also well-known pragmatic problems in any
>multilevel scheduling scheme where the levels proceed without knowledge
>of each other.
>
>> In "From EROS to Coyotos/BitC: Open Source Meets Open Proofs"
>> 
>> he says:
>> _______________________________________________________
>> The Coyotos effort inherits two useful successes
>> from the EROS effort:
>> 
>>     A verification proof that the architecture can en-
>>     force a security property known as “confinement.”
>> and ...
>> _______________________________________________________
>> 
>> but then in "Verifying the EROS Confinement Mechanism" he says:
>> 
>> "In this paper, we will ignore issues of covert channels.  While
>important,
>> reducing such channels is of interest only when it has been shown that
>overt
>> channels have been closed."
>
>I'm reminded of Talmud scholars, who pick up a bit here and a bit there
>and draw whatever conclusion they want. Not that Jed has done so here,
>but the style match was pretty good. :-)
>
>Confession: yes, I think the covert channel question is preempted by the
>overt channel question, but that wasn't the reason for the disclaimer.
>The disclaimer was added to satisfy academic reviewers.

This reminds me of something I wrote some year ago about automation:

"it is clear that automating system tuning is not as simple as automating
the batch work. The typical path is to automate at first the mechanical
and recurring operations, and later to extend the automatisms to more
"intelligent" operations."  (http://www.selnet.org/pubs/selbkgnd.html)

The topic is quite different, but the logical approach is the same: at
first address the overt channel question and later extend to the covert
channel question (which is more difficult), so at the end, I agree with
Shap.

>
>The factory is significant work. It is a unique combination of
>confinement and isolation that serves as a building block for larger
>security architectures. Proving the correctness of the criteria was
>non-trivial. All of this, in my mind, is pretty clear, and warranted
>publication of the result. Later, I would realize that the result also
>provided a verification gold standard for overall system verification,
>though this wasn't clear to me at the time.
>
>But by far the greatest objection of reviewers was over an issue of
>definition. Almost universally the reviewers complained that Lampson
>included covert communication in his definition of confinement. Since
>the paper didn't, they argued, it didn't solve the confinement problem
>and must not claim to solve the confinement problem (i.e. the property
>needed to be renamed). On these grounds the paper was heatedly argued by
>the program committee and was nearly rejected.
>
>Now I have several issues with this. First, Lampson himself doesn't take
>that paper all that seriously. He is very clear that it was a workshop
>paper sketching a problem, as opposed to any rigorous definition.
>Second, the techniques appropriate to covert suppression are entirely
>orthogonal to the overt authority issues; in my opinion Lampson erred by
>conflating them in his definition of confinement. Third and finally, the
>tone of the reviews was simply pissy, being more focused on preserving
>the integrity of a non-rigorous definition than on evaluating the merit
>of the work at hand.
>
>>From my perspective it was actually important to hijack the definition
>of confinement. Ignoring the conflation of covert channels, Lampson's
>characterization was pretty good, and it was a widely known label for a
>foundational property.

I find this approach of solving problems step by step, much more practical
and easy to afford. The fact that Lampson conflated overt and covert
channels in his definition of confinement was kind of a jump over two
steps. Having followed Shap's work, it is now pretty clear to me that the
conflation was a mistake.

>
>Subsequent papers, including Chander Dean and Mitchell's
>"State-Transition Model of Trust Management and Access Control" have
>implicitly relied on our (overt) definition of confinement. Most have
>failed to cite us for it. The CDM paper is particularly amusing, because
>it reasons carefully from a badly constructed abstraction to conclude
>that confinement in capability systems is impossible, failing to even
>address the existence of our verification result. The CCS committee was
>asleep at the switch that year.
>
>So the qualifier went in purely to satisfy the reviewers.
>
>
>> Sure I know of many of the commercial projects and even
>> products based on capabilities (e.g. going back to the
>> Plessey 250), but such systems (hardware and software)
>> have been and continue to be commercially insignificant.
>
>One should not discount the AS/400 as commercially insignificant.
>
>> Please don't oversell capabilities and suggest that they can
>> solve covert channel problems like wall banging.  I think it's enough
>> to deal with overt communication channels as Jonathan suggests.
>
>I agree -- on both points.

I think we agree on this topic. It was never my intention to oversell
capabilities, it was my intention to say that covert channel problems have
been addressed in the sense that they have been discussed (at least) in
this list, and that I have noticed some countermeasures in the ASPOS/PP.


>
>> > >>I was suggesting to make the login process strictly tied to the
>> > >>kernel model of operation.
>> > >
>> > >I'm afraid I still don't understand what you mean by the "kernel
>model of
>> > >operation."
>> >
>> >The meaning is Atomic action kernel: setup phase -> commit point ->
>action
>> >phase.
>> >The action phase either succeeds entirely or fails entirely, this is
the
>> >notion of "atomic" as Shap defines it.
>
>I think I can claim credit for the coinage "atomic action kernel". I
>certainly don't deserve credit for coining "atomic". :-) At a guess, the
>coinage of "atomic" probably predates quantum mechanics.

Shap:
  Yes, I have "stolen" the term from your paper :-)

>
>> Fine.  I don't see what the term "kernel" adds to the notion of an
atomic
>> operation, but I don't think it matters.
>
>Kernel isn't adding to atomic. Atomic is adding to kernel.
>
>It adds because the prevailing models in the OS literature are either
>threaded kernels or interrupt driven kernels, neither of which has the
>same properties w.r.t. management of state. The idea of an atomic action
>kernel used to be very common, and remains so in industrial control
>contexts, but was never much favored in the general-purpose OS world.
>The term exists primarily to give this architectural class of kernels a
>differentiating label.
>
>
>shap
>
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