linux-kernel - Re: [PATCH v1 3/3] KEYS: trusted: Introduce support for NXP CAAM-based trusted keys

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Date:   Wed, 24 Mar 2021 09:14:02 -0700
From:   James Bottomley <jejb@...ux.ibm.com>
To:     Mimi Zohar <zohar@...ux.ibm.com>,
        Ahmad Fatoum <a.fatoum@...gutronix.de>,
        Horia Geantă <horia.geanta@....com>,
        Jonathan Corbet <corbet@....net>,
        David Howells <dhowells@...hat.com>,
        Jarkko Sakkinen <jarkko@...nel.org>
Cc:     "kernel@...gutronix.de" <kernel@...gutronix.de>,
        James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Aymen Sghaier <aymen.sghaier@....com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "David S. Miller" <davem@...emloft.net>,
        Udit Agarwal <udit.agarwal@....com>,
        Jan Luebbe <j.luebbe@...gutronix.de>,
        David Gstir <david@...ma-star.at>,
        Franck Lenormand <franck.lenormand@....com>,
        Sumit Garg <sumit.garg@...aro.org>,
        "keyrings@...r.kernel.org" <keyrings@...r.kernel.org>,
        "linux-crypto@...r.kernel.org" <linux-crypto@...r.kernel.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-integrity@...r.kernel.org" <linux-integrity@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-security-module@...r.kernel.org" 
        <linux-security-module@...r.kernel.org>
Subject: Re: [PATCH v1 3/3] KEYS: trusted: Introduce support for NXP
 CAAM-based trusted keys

On Tue, 2021-03-23 at 14:07 -0400, Mimi Zohar wrote:
> On Tue, 2021-03-23 at 17:35 +0100, Ahmad Fatoum wrote:
> > Hello Horia,
> > 
> > On 21.03.21 21:48, Horia Geantă wrote:
> > > On 3/16/2021 7:02 PM, Ahmad Fatoum wrote:
> > > [...]
> > > > +struct trusted_key_ops caam_trusted_key_ops = {
> > > > +	.migratable = 0, /* non-migratable */
> > > > +	.init = trusted_caam_init,
> > > > +	.seal = trusted_caam_seal,
> > > > +	.unseal = trusted_caam_unseal,
> > > > +	.exit = trusted_caam_exit,
> > > > +};
> > > caam has random number generation capabilities, so it's worth
> > > using that
> > > by implementing .get_random.
> > 
> > If the CAAM HWRNG is already seeding the kernel RNG, why not use
> > the kernel's?
> > 
> > Makes for less code duplication IMO.
> 
> Using kernel RNG, in general, for trusted keys has been discussed
> before.   Please refer to Dave Safford's detailed explanation for not
> using it [1].
> 
> thanks,
> 
> Mimi
> 
> [1] 
> https://lore.kernel.org/linux-integrity/BCA04D5D9A3B764C9B7405BBA4D4A3C035F2A38B@ALPMBAPA12.e2k.ad.ge.com/

I still don't think relying on one source of randomness to be
cryptographically secure is a good idea.  The fear of bugs in the
kernel entropy pool is reasonable, but since it's widely used they're
unlikely to persist very long.  Studies have shown that some TPMs
(notably the chinese manufactured ones) have suspicious failures in
their RNGs:

https://www.researchgate.net/publication/45934562_Benchmarking_the_True_Random_Number_Generator_of_TPM_Chips

And most cryptograhpers recommend using a TPM for entropy mixing rather
than directly:

https://blog.cryptographyengineering.com/category/rngs/

The TPMFail paper also shows that in spite of NIST certification
things can go wrong with a TPM:

https://tpm.fail/

James