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

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Message-ID: <20201123171800.GA6407@duo.ucw.cz>
Date:   Mon, 23 Nov 2020 18:18:00 +0100
From:   Pavel Machek <pavel@....cz>
To:     Mimi Zohar <zohar@...ux.ibm.com>
Cc:     Tushar Sugandhi <tusharsu@...ux.microsoft.com>,
        stephen.smalley.work@...il.com, casey@...aufler-ca.com,
        agk@...hat.com, snitzer@...hat.com, gmazyland@...il.com,
        paul@...l-moore.com, tyhicks@...ux.microsoft.com,
        sashal@...nel.org, jmorris@...ei.org, nramas@...ux.microsoft.com,
        linux-integrity@...r.kernel.org, selinux@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        linux-kernel@...r.kernel.org, dm-devel@...hat.com
Subject: Re: [PATCH v6 0/8] IMA: support for measuring kernel integrity
 critical data

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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