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

[Jarkko Sakkinen]

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?

/Jarkko