Re: [PATCH] KEYS: asym_tpm: Switch to get_random_bytes()

[Jarkko Sakkinen]

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@xxxxxxxxxxxxxxxxxxxxx/
> > > 
> > > 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?

/Jarkko