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Date:   Fri, 4 Oct 2019 21:30:03 +0300
From:   Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
To:     "Safford, David (GE Global Research, US)" <david.safford@ge.com>
Cc:     Mimi Zohar <zohar@linux.ibm.com>,
        "Wiseman, Monty (GE Global Research, US)" <monty.wiseman@ge.com>,
        "linux-integrity@vger.kernel.org" <linux-integrity@vger.kernel.org>,
        "stable@vger.kernel.org" <stable@vger.kernel.org>,
        David Howells <dhowells@redhat.com>,
        Herbert Xu <herbert@gondor.apana.org.au>,
        "David S. Miller" <davem@davemloft.net>,
        "open list:ASYMMETRIC KEYS" <keyrings@vger.kernel.org>,
        "open list:CRYPTO API" <linux-crypto@vger.kernel.org>,
        open list <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] KEYS: asym_tpm: Switch to get_random_bytes()
Message-ID: <20191004183003.GD6945@linux.intel.com>
References: <20190926171601.30404-1-jarkko.sakkinen@linux.intel.com>
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 <1570107752.4421.183.camel@linux.ibm.com>
 <20191003175854.GB19679@linux.intel.com>
 <1570128827.5046.19.camel@linux.ibm.com>
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> > There are many good reasons for wanting the keys to be based on the
> > TPM generator.  As the source for the kernel random number generator
> > itself says, some systems lack good randomness at startup, and systems
> > should preserve and reload the pool across shutdown and startup.
> > There are use cases for trusted keys which need to generate keys 
> > before such scripts have run. Also, in some use cases, we need to show
> > that trusted keys are FIPS compliant, which is possible with TPM
> > generated keys.
> 
> If you are able to call tpm_get_random(), the driver has already
> registered TPN as hwrng. With this solution you fail to follow the
> principle of defense in depth. If the TPM random number generator
> is compromissed (has a bug) using the entropy pool will decrease
> the collateral damage.

I.e. you make everything depend on single point of failure instead
of multiple (e.g. rdrand, TPM, whatnot).

/Jarkko