linux-kernel - Re: [PATCH] KEYS: asym_tpm: Switch to get_random

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Message-Id: <1570128827.5046.19.camel@linux.ibm.com>
Date:   Thu, 03 Oct 2019 14:53:47 -0400
From:   Mimi Zohar <zohar@...ux.ibm.com>
To:     Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        David Safford <david.safford@...com>
Cc:     linux-integrity@...r.kernel.org, stable@...r.kernel.org,
        David Howells <dhowells@...hat.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "David S. Miller" <davem@...emloft.net>,
        "open list:ASYMMETRIC KEYS" <keyrings@...r.kernel.org>,
        "open list:CRYPTO API" <linux-crypto@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] KEYS: asym_tpm: Switch to get_random_bytes()

[Cc'ing David Safford]

On Thu, 2019-10-03 at 20:58 +0300, Jarkko Sakkinen wrote:
> On Thu, Oct 03, 2019 at 09:02:32AM -0400, Mimi Zohar wrote:
> > On Thu, 2019-10-03 at 14:41 +0300, Jarkko Sakkinen wrote:
> > > On Wed, Oct 02, 2019 at 10:00:19AM -0400, Mimi Zohar wrote:
> > > > On Thu, 2019-09-26 at 20:16 +0300, Jarkko Sakkinen wrote:
> > > > > Only the kernel random pool should be used for generating random numbers.
> > > > > TPM contributes to that pool among the other sources of entropy. In here it
> > > > > is not, agreed, absolutely critical because TPM is what is trusted anyway
> > > > > but in order to remove tpm_get_random() we need to first remove all the
> > > > > call sites.
> > > > 
> > > > At what point during boot is the kernel random pool available?  Does
> > > > this imply that you're planning on changing trusted keys as well?
> > > 
> > > Well trusted keys *must* be changed to use it. It is not a choice
> > > because using a proprietary random number generator instead of defacto
> > > one in the kernel can be categorized as a *regression*.
> > 
> > I really don't see how using the TPM random number for TPM trusted
> > keys would be considered a regression.  That by definition is a
> > trusted key.  If anything, changing what is currently being done would
> > be the regression. 
> 
> It is really not a TPM trusted key. It trusted key that gets sealed with
> the TPM. The key itself is used in clear by kernel. The random number
> generator exists in the kernel to for a reason.
> 
> It is without doubt a regression.

You're misusing the term "regression" here.  A regression is something
that previously worked and has stopped working.  In this case, trusted
keys has always been based on the TPM random number generator.  Before
changing this, there needs to be some guarantees that the kernel
random number generator has a pool of random numbers early, on all
systems including embedded devices, not just servers.

Mimi