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

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Message-ID: <20191015170450.GB5444@ziepe.ca>
Date:   Tue, 15 Oct 2019 14:04:50 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
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 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?

Jason