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

From: Fan Wu <wufan@linux.microsoft.com>
To: Roberto Sassu <roberto.sassu@huaweicloud.com>
Cc: axboe@kernel.dk, linux-block@vger.kernel.org, tytso@mit.edu,
	dm-devel@redhat.com, corbet@lwn.net, roberto.sassu@huawei.com,
	Deven Bowers <deven.desai@linux.microsoft.com>,
	linux-doc@vger.kernel.org, snitzer@kernel.org, jmorris@namei.org,
	zohar@linux.ibm.com, linux-kernel@vger.kernel.org,
	ebiggers@kernel.org, linux-security-module@vger.kernel.org,
	linux-audit@redhat.com, eparis@redhat.com,
	linux-fscrypt@vger.kernel.org, linux-integrity@vger.kernel.org,
	agk@redhat.com, serge@hallyn.com
Subject: Re: [RFC PATCH v9 03/16] ipe: add evaluation loop and introduce 'boot_verified' as a trust provider
Date: Fri, 10 Feb 2023 15:21:54 -0800	[thread overview]
Message-ID: <20230210232154.GA17962@linuxonhyperv3.guj3yctzbm1etfxqx2vob5hsef.xx.internal.cloudapp.net> (raw)
In-Reply-To: <061df661004a06ef1e8790d48157c7ba4ecfc009.camel@huaweicloud.com>

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@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
> 

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