[cap-talk] Capabilities - the rub, an account

Jed at Webstart donnelley1 at webstart.com
Sun Nov 26 15:02:23 CST 2006


At 05:14 AM 11/24/2006, Marcus Brinkmann wrote:
>On Fri, 2006-11-24 at 16:17 +0800, John McCabe-Dansted wrote:
>...
>Capability systems usually solve this issue by foregoing the goal of
>collecting and maintaining global information about what's going on in
>the system.  I agree with your finer point that at some level you want
>to re-establish such information at least at an intermediate level, for
>example user-to-user interaction.

I believe this is where I've been trying to take this thread.

>I think that this makes sense, and it is one reason why I consider
>revocable copies an essential feature in capability systems.
>Unfortunately, building a system that has unlimited (by the system)
>nesting of capability wrappers and no denial-of-service threat scenario
>is a big challenge (see the current L4 redesign efforts with
>user-managed kernel memory).

I don't think such an implementation is as difficult as it might seem,
particularly if the protocol for communicating permission goes through
the server of the 'object'.  In that case what you consider a "big challenge"
I believe becomes rather simple book keeping.

>In general one would want to reduce the role of the system administrator
>and the system software in general in a modern operating system.  The
>Unix model of root on the one side and the users on the other side has
>great disadvantages, both in terms of security and in terms of
>functionality.

I agree, though I go further to say that the whole Mandatory Access
Control model is broken and even the discretional access control
mechanisms designed to that philosophy (as in Unix) are so
unwieldy as to really deserve Lampson's castigation, for example
(stealing his words, something like):

<humor>
ACLs have done an enormous amount of damage to security because
they require you to do all your access delegations through a systems
administrator.  There is so much overhead in doing so, that, even if
you get right today it will be wrong three months from now.  Nobody will
have the patience to ever look at it again because there is just too
much overhead going through the systems administrator for every
change.  So I say absolutely no to administrator controlled ACLs.
Everything should be as automated and as fine grained (O-O) as
possible so that people generally don't have to deal with delegations.
That's a very unpopular position with most people.  I think there's a lot
of empirical evidence that tells us now that it's right.
</humor>

--Jed http://www.webstart.com/jed/ 




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