Re: [PATCH] tpm: Salt tpm_get_random() result with get_random_bytes()

[Jarkko Sakkinen]

On Wed, Oct 16, 2019 at 01:09:08PM -0300, Jason Gunthorpe wrote:
> On Wed, Oct 16, 2019 at 01:43:22PM +0300, Jarkko Sakkinen wrote:
> > On Wed, Oct 16, 2019 at 01:38:05PM +0300, Jarkko Sakkinen wrote:
> > > On Tue, Oct 15, 2019 at 02:04:50PM -0300, Jason Gunthorpe wrote:
> > > > On Tue, Oct 15, 2019 at 03:47:02PM +0300, Jarkko Sakkinen wrote:
> > > > > Salt the result that comes from the TPM RNG with random bytes from the
> > > > > kernel RNG. This will allow to use tpm_get_random() as a substitute for
> > > > > get_random_bytes().  TPM could have a bug (making results predicatable),
> > > > > backdoor or even an inteposer in the bus. Salting gives protections
> > > > > against these concerns.
> > > > 
> > > > Seems like a dangerous use case, why would any kernel user that cared
> > > > about quality of randomness ever call a tpm_* API to get quality
> > > > random data?
> > > 
> > > This is related to this discussion:
> > > 
> > > https://lore.kernel.org/linux-integrity/CAE=NcrY3BTvD-L2XP6bsO=9oAJLtSD0wYpUymVkAGAnYObsPzQ@xxxxxxxxxxxxxx/T/#t
> > > 
> > > I could also move this to the call site.
> > 
> > But I hear you anyway.
> > 
> > I think for trusted keys the best strategy would be to do
> > exactly this:
> > 
> > 1. Generate one random value with get_random_bytes_arch()
> > 2. Generate another with backend specific technology (we
> >    have now two TPM and TEE) if an RNG available.
> > 3. Xor the values together.
> 
> Feels like something the random core should handle - maybe some way to
> say 'my trust model requires trust in this RNG' and then the random
> core can more heavily weight data from that RNG

Yeah, I think. I'll study these emails threads and RNG implementation
a bit when I have more time. Now I think I lack some knowledge to say
anything educated so better to take a step back and go back to the
drawing board.

Thank you for the suggestion.

/Jarkko