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

[Jarkko Sakkinen]

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.

/Jarkko