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

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Message-ID: <20191021113939.GA11649@linux.intel.com>
Date:   Mon, 21 Oct 2019 14:39:39 +0300
From:   Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
To:     James Bottomley <James.Bottomley@...senPartnership.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 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@HansenPartnership.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.

/Jarkko