linux-kernel - Re: [PATCH] tpm: Salt tpm_get_random() result with get_random

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Message-ID: <20191016104322.GC10184@linux.intel.com>
Date:   Wed, 16 Oct 2019 13:43:22 +0300
From:   Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     linux-integrity@...r.kernel.org,
        David Safford <david.safford@...com>,
        Pascal Van Leeuwen <pvanleeuwen@...imatrix.com>,
        James Bottomley <James.Bottomley@...senPartnership.com>,
        Peter Huewe <peterhuewe@....de>, Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] tpm: Salt tpm_get_random() result with get_random_bytes()

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@mail.gmail.com/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