Re: [PATCH v6 0/8] IMA: support for measuring kernel integrity critical data

From: Pavel Machek <pavel@ucw.cz>
To: Mimi Zohar <zohar@linux.ibm.com>
Cc: Tushar Sugandhi <tusharsu@linux.microsoft.com>,
	stephen.smalley.work@gmail.com, casey@schaufler-ca.com,
	agk@redhat.com, snitzer@redhat.com, gmazyland@gmail.com,
	paul@paul-moore.com, tyhicks@linux.microsoft.com,
	sashal@kernel.org, jmorris@namei.org, nramas@linux.microsoft.com,
	linux-integrity@vger.kernel.org, selinux@vger.kernel.org,
	linux-security-module@vger.kernel.org,
	linux-kernel@vger.kernel.org, dm-devel@redhat.com
Subject: Re: [PATCH v6 0/8] IMA: support for measuring kernel integrity critical data
Date: Mon, 23 Nov 2020 18:18:00 +0100	[thread overview]
Message-ID: <20201123171800.GA6407@duo.ucw.cz> (raw)
In-Reply-To: <d82ad1cac36e948c904300548c64244c145589ee.camel@linux.ibm.com>

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Hi!

> > > >How is it supposed to be useful?
> > > >
> > > >I'm pretty sure there are critical data that are not measured by
> > > >proposed module... and that are written under normal circumstances.
> > > >
> > > The goal of this series is to introduce the IMA hook
> > > measure_critical_data() and the necessary policies to use it; and
> > > illustrate that use with one example (SELinux). It is not scalable to
> > > identify and update all the critical data sources to use the proposed
> > > module at once.
> > > 
> > > A piecemeal approach to add more critical data measurement in subsequent
> > > patches would be easy to implement and review.
> > 
> > Basically every other data structure in kernel is "critical" by your
> > definition, and you can't really measure them all; some of them change
> > rather often. Going piecemeal does not really help here.
> 
> Agreed, measuring data structures that change is not really applicable.
> However, measuring data structures that once initialized don't change,
> does make sense (similar concept to __ro_after_init).  The attestation
> server doesn't need to know anything about the measurement, other than
> more than a single measurement is indicative of a problem.

So, why not simply measure everything that is ro_after_init?

But... I really fail to see how this is useful. It is trivial to
"backdoor" kernel w/o modifying anything that is
ro_after_init. (Example: page tables).

								Pavel
-- 
http://www.livejournal.com/~pavelmachek

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