[RFC PATCH v9 03/16] ipe: add evaluation loop and introduce 'boot_verified' as a trust provider

[Fan Wu]

On Tue, Jan 31, 2023 at 04:49:44PM +0100, Roberto Sassu wrote:
> On Mon, 2023-01-30 at 14:57 -0800, Fan Wu wrote:
> > From: Deven Bowers <deven.desai at linux.microsoft.com>
> > 
> > IPE must have a centralized function to evaluate incoming callers
> > against IPE's policy. This iteration of the policy against the rules
> > for that specific caller is known as the evaluation loop.
> 
> Not sure if you check the properties at every access.
> 
> >From my previous comments (also for previous versions of the patches)
> you could evaluate the property once, by calling the respective
> functions in the other subsystems.
> 
> Then, you reserve space in the security blob for inodes and superblocks
> to cache the decision. The format could be a policy sequence number, to
> ensure that the cache is valid only for the current policy, and a bit
> for every hook you enforce.

Thanks for raising this idea. I agree that if the property evaluation
leads to a performance issue, it will be better to cache the evaluation
result. But for this version, all the property evaluations are simple,
so it is just as fast as accessing a cache. Also, for the initial
version we prefer to keep the patch as minimal as possible. 

If the policy evolved to be super complex and the evaluation becomes
a bottleneck, cache support will absolutely be the right way we will go.
-Fan

> 
> Also, currently you rely on the fact that the properties you defined
> are immutable and the immutability is guaranteed by the other
> subsystems, so no write can occur.
> 
> But if you remove this limitation, the immutability is not guaranteed
> anymore by the other subsystems (for example if a file is in an ext4
> filesystem), the LSM needs to take extra care to ensure that the
> properties are still verified. This would be required for example if
> IPE is used in conjuction with DIGLIM.
> 
> In my opinion, IPE value would increase if the generic enforcement
> mechanism is property-agnostic.
> 
> Roberto
>