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

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Message-ID: <20191016104110.GB10184@linux.intel.com>
Date:   Wed, 16 Oct 2019 13:41:10 +0300
From:   Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
To:     Janne Karhunen <janne.karhunen@...il.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>,
        Jason Gunthorpe <jgg@...pe.ca>, Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        open list <linux-kernel@...r.kernel.org>,
        Mimi Zohar <zohar@...ux.ibm.com>
Subject: Re: [PATCH] tpm: Salt tpm_get_random() result with get_random_bytes()

On Wed, Oct 16, 2019 at 10:02:01AM +0300, Janne Karhunen wrote:
> On Tue, Oct 15, 2019 at 3:50 PM Jarkko Sakkinen
> <jarkko.sakkinen@...ux.intel.com> 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.
> 
> The current issue in the randomness from my point of view is that
> encrypted filesystems, ima etc in common deployments require high
> quality entropy just few seconds after the system has powered on for
> the first time. It is likely that people want to keep their keys
> device specific, so the keys need to be generated on the first boot
> before any of the filesystems mount.

This patch does not have the described issue.

Which call sites are you talking about?

/Jarkko