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linux-security-module: Re: [Apparmor-dev] Re: File descriptor ob

Re: [Apparmor-dev] Re: File descriptor object capability LSM module. Feasability?

From: Mark Seaborn <mrs_at_nospam>
Date: Sat Nov 10 2007 - 13:20:35 GMT

Crispin Cowan <> wrote:

> The other issue with the object capability model is analyzability.
> Stephen Smalley complained about this in some public setting a while ago
> when someone basically asked for an object capability enhancement to
> SELinux. Stephen is correct, in that with a pure ambient capability
> model, you can analyze the text of the complete system policy, and
> readily determine the maximum permissions that any given entity will
> have under that policy. With an object capability model, the scope of
> access of a given program is determined by what gets passed into it, and
> so you would have to resort to tools to compute the transitive closure
> of all capabilities that *could* be delegated to it.

That is not true in the general case. If process A and process B can communicate, process A can delegate any authority it has to process B, if both processes want to do that. Putting object-capability support into the kernel is just a way of making that more efficient and convenient. SELinux/Apparmor are no more analyzable than object-capability systems.

There is only a difference in analyzability if:

  • you decide you're not including malicious programs (i.e. not an adversarial model), or
  • you define analyzability so that you're measuring something that is not important (permissions) rather than something that is (authority).

> o Show a use case of a real program (not a straw man) such
> that with the current features, your choice is to either
> provide a dangerously permissive security policy, or the
> policy breaks the program.

An e-mail client. With a static security system, if you want to be able to use the e-mail program to send attachments, you must grant the e-mail program access to all of your files in advance, because you could want to send any file as an attachment.

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