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

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Message-ID: <1572361096.4812.3.camel@HansenPartnership.com>
Date:   Tue, 29 Oct 2019 07:58:16 -0700
From:   James Bottomley <James.Bottomley@...senPartnership.com>
To:     Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
Cc:     "Safford, David (GE Global Research, US)" <david.safford@...com>,
        Ken Goldman <kgold@...ux.ibm.com>,
        Mimi Zohar <zohar@...ux.ibm.com>,
        "linux-integrity@...r.kernel.org" <linux-integrity@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        "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 Tue, 2019-10-29 at 10:42 +0200, Jarkko Sakkinen wrote:
> On Mon, Oct 21, 2019 at 02:39:39PM +0300, Jarkko Sakkinen wrote:
> > On Thu, Oct 17, 2019 at 09:04:40PM +0300, Jarkko Sakkinen wrote:
> > > On Wed, Oct 16, 2019 at 03:10:29PM -0400, James Bottomley wrote:
> > > > On Wed, 2019-10-16 at 19:25 +0300, Jarkko Sakkinen wrote:
> > > > > On Wed, Oct 16, 2019 at 08:34:12AM -0400, James Bottomley
> > > > > wrote:
> > > > > > reversible ciphers are generally frowned upon in random
> > > > > > number
> > > > > > generation, that's why the krng uses chacha20.  In general
> > > > > > I think
> > > > > > we shouldn't try to code our own mixing and instead should
> > > > > > get the
> > > > > > krng to do it for us using whatever the algorithm du jour
> > > > > > that the
> > > > > > crypto guys have blessed is.  That's why I proposed adding
> > > > > > the TPM
> > > > > > output to the krng as entropy input and then taking the
> > > > > > output of
> > > > > > the krng.
> > > > > 
> > > > > It is already registered as hwrng. What else?
> > > > 
> > > > It only contributes entropy once at start of OS.
> > > 
> > > Ok.
> > > 
> > > > >  Was the issue that it is only used as seed when the rng is
> > > > > init'd
> > > > > first? I haven't at this point gone to the internals of krng.
> > > > 
> > > > Basically it was similar to your xor patch except I got the
> > > > kernel rng
> > > > to do the mixing, so it would use the chacha20 cipher at the
> > > > moment
> > > > until they decide that's unsafe and change it to something
> > > > else:
> > > > 
> > > > https://lore.kernel.org/linux-crypto/1570227068.17537.4.camel@H
> > > > ansenPartnership.com/
> > > > 
> > > > It uses add_hwgenerator_randomness() to do the mixing.  It also
> > > > has an
> > > > unmixed source so that read of the TPM hwrng device works as
> > > > expected.
> > > 
> > > Thinking that could this potentially racy? I.e. between the calls
> > > something else could eat the entropy added?
> > 
> > Also, what is wrong just taking one value from krng and mixing
> > it with a value from TPM RNG where needed? That would be non-racy
> > too.
> 
> I guess we can move forward with this?

Sure I suppose; can we can figure out how to get the mixing function du
jour exposed?

James