linux-kernel - Re: [PATCH v10 2/7] KEYS: trusted: allow use of kernel RNG for key material

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Date:   Tue, 17 May 2022 21:10:57 +0300
From:   Jarkko Sakkinen <jarkko@...nel.org>
To:     "Jason A. Donenfeld" <Jason@...c4.com>,
        Ahmad Fatoum <a.fatoum@...gutronix.de>
Cc:     James Bottomley <jejb@...ux.ibm.com>,
        Mimi Zohar <zohar@...ux.ibm.com>,
        David Howells <dhowells@...hat.com>, kernel@...gutronix.de,
        Sumit Garg <sumit.garg@...aro.org>,
        Pankaj Gupta <pankaj.gupta@....com>,
        David Gstir <david@...ma-star.at>,
        Michael Walle <michael@...le.cc>,
        John Ernberg <john.ernberg@...ia.se>,
        James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Horia Geantă <horia.geanta@....com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "David S. Miller" <davem@...emloft.net>,
        Jan Luebbe <j.luebbe@...gutronix.de>,
        Eric Biggers <ebiggers@...nel.org>,
        Richard Weinberger <richard@....at>,
        Franck LENORMAND <franck.lenormand@....com>,
        Matthias Schiffer <matthias.schiffer@...tq-group.com>,
        keyrings@...r.kernel.org, linux-crypto@...r.kernel.org,
        linux-integrity@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-security-module@...r.kernel.org
Subject: Re: [PATCH v10 2/7] KEYS: trusted: allow use of kernel RNG for key
 material

On Tue, 2022-05-17 at 19:27 +0200, Jason A. Donenfeld wrote:
> On Fri, May 13, 2022 at 04:57:00PM +0200, Ahmad Fatoum wrote:
> > +       trusted.rng=    [KEYS]
> > +                       Format: <string>
> > +                       The RNG used to generate key material for trusted keys.
> > +                       Can be one of:
> > +                       - "kernel"
> > +                       - the same value as trusted.source: "tpm" or "tee"
> > +                       - "default"
> > +                       If not specified, "default" is used. In this case,
> > +                       the RNG's choice is left to each individual trust source.
> > +
> 
> As a general mechanism, I object to this. The kernel's RNG must be
> trusted in the first place for key material. That's the whole point of
> it.

I would relax this  a bit: kernel's RNG must be implicitly must be
trusted. If the parameter is used, you make an explicit choice that
you are aware of the trust.

If this was opt-out parameter, instead of opt-in, I would get your
argument.

> However, it sounds like you're not proposing a general mechanism, but
> just something particular to this "trusted keys" business. In that case,
> this should be a module flag, and thus not documented here, but rather
> some place namespaced to your trusted keys stuff. "trusted_keys.preferred_rng={whatever}"

However, I think this a good proposal. Let's make it a module parameter
instead.

> Jason

BR, Jarkko