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

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Message-Id: <1570140491.5046.33.camel@linux.ibm.com>
Date:   Thu, 03 Oct 2019 18:08:11 -0400
From:   Mimi Zohar <zohar@...ux.ibm.com>
To:     Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
Cc:     David Safford <david.safford@...com>,
        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()

On Fri, 2019-10-04 at 00:57 +0300, Jarkko Sakkinen wrote:
> On Fri, Oct 04, 2019 at 12:51:25AM +0300, Jarkko Sakkinen wrote:
> > On Thu, Oct 03, 2019 at 02:53:47PM -0400, Mimi Zohar wrote:
> > > [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.
> > 
> > I'm not using the term regression incorrectly here. Wrong function
> > was used to generate random numbers for the payload here. It is an
> > obvious bug.
> 
> At the time when trusted keys was introduced I'd say that it was a wrong
> design decision and badly implemented code. But you are right in that as
> far that code is considered it would unfair to speak of a regression.
> 
> asym-tpm.c on the other hand this is fresh new code. There has been
> *countless* of discussions over the years that random numbers should
> come from multiple sources of entropy. There is no other categorization
> than a bug for the tpm_get_random() there.

This week's LWN article on "5.4 Merge window, part 2" discusses "boot-
time entropy".  This article couldn't have been more perfectly timed.

Mimi