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

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Message-ID: <20191016103805.GA10184@linux.intel.com>
Date:   Wed, 16 Oct 2019 13:38:05 +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 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.

/Jarkko