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Subject: Re: [PATCH] KEYS: asym_tpm: Switch to get_random_bytes()
From:   James Bottomley <James.Bottomley@HansenPartnership.com>
To:     Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Cc:     Mimi Zohar <zohar@linux.ibm.com>,
        David Safford <david.safford@ge.com>,
        linux-integrity@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>
Date:   Fri, 04 Oct 2019 11:24:51 -0700
In-Reply-To: <20191004182216.GB6945@linux.intel.com>
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On Fri, 2019-10-04 at 21:22 +0300, Jarkko Sakkinen wrote:
> On Thu, Oct 03, 2019 at 04:59:37PM -0700, James Bottomley wrote:
> > I think the principle of using multiple RNG sources for strong keys
> > is a sound one, so could I propose a compromise:  We have a tpm
> > subsystem random number generator that, when asked for <n> random
> > bytes first extracts <n> bytes from the TPM RNG and places it into
> > the kernel entropy pool and then asks for <n> random bytes from the
> > kernel RNG? That way, it will always have the entropy to satisfy
> > the request and in the worst case, where the kernel has picked up
> > no other entropy sources at all it will be equivalent to what we
> > have now (single entropy source) but usually it will be a much
> > better mixed entropy source.
> 
> I think we should rely the existing architecture where TPM is
> contributing to the entropy pool as hwrng.

That doesn't seem to work: when I trace what happens I see us inject 32
bytes of entropy at boot time, but never again.  I think the problem is
the kernel entropy pool is push not pull and we have no triggering
event in the TPM to get us to push.  I suppose we could set a timer to
do this or perhaps there is a pull hook and we haven't wired it up
correctly?

James