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

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Message-ID: <20191017162141.GA6667@linux.intel.com>
Date:   Thu, 17 Oct 2019 19:21:41 +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 01:09:08PM -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@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.
> 
> 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