linux-kernel - Re: [RFC PATCH v9 03/16] ipe: add evaluation loop and introduce 'boot

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Date:   Fri, 10 Feb 2023 15:21:54 -0800
From:   Fan Wu <wufan@...ux.microsoft.com>
To:     Roberto Sassu <roberto.sassu@...weicloud.com>
Cc:     corbet@....net, zohar@...ux.ibm.com, jmorris@...ei.org,
        serge@...lyn.com, tytso@....edu, ebiggers@...nel.org,
        axboe@...nel.dk, agk@...hat.com, snitzer@...nel.org,
        eparis@...hat.com, paul@...l-moore.com, linux-doc@...r.kernel.org,
        linux-integrity@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        linux-fscrypt@...r.kernel.org, linux-block@...r.kernel.org,
        dm-devel@...hat.com, linux-audit@...hat.com,
        roberto.sassu@...wei.com, linux-kernel@...r.kernel.org,
        Deven Bowers <deven.desai@...ux.microsoft.com>
Subject: Re: [RFC PATCH v9 03/16] ipe: add evaluation loop and introduce
 'boot_verified' as a trust provider

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@...ux.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
>