linux-kernel - Re: [PATCH] KEYS: asym_tpm: Switch to get_random

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Message-ID: <20191016110031.GE10184@linux.intel.com>
Date:   Wed, 16 Oct 2019 14:00:31 +0300
From:   Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
To:     James Bottomley <James.Bottomley@...senPartnership.com>
Cc:     "Safford, David (GE Global Research, US)" <david.safford@...com>,
        Ken Goldman <kgold@...ux.ibm.com>,
        Mimi Zohar <zohar@...ux.ibm.com>,
        "linux-integrity@...r.kernel.org" <linux-integrity@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        "open list:ASYMMETRIC KEYS" <keyrings@...r.kernel.org>,
        "open list:CRYPTO API" <linux-crypto@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] KEYS: asym_tpm: Switch to get_random_bytes()

On Mon, Oct 14, 2019 at 12:29:57PM -0700, James Bottomley wrote:
> The job of the in-kernel rng is simply to produce a mixed entropy pool
> from which we can draw random numbers.  The idea is that quite a few
> attackers have identified the rng as being a weak point in the security
> architecture of the kernel, so if we mix entropy from all the sources
> we have, you have to compromise most of them to gain some predictive
> power over the rng sequence.

The documentation says that krng is suitable for key generation.
Should the documentation changed to state that it is unsuitable?

> The point is not how certified the TPM RNG is, the point is that it's a
> single source and if we rely on it solely for some applications, like
> trusted keys, then it gives the attackers a single known point to go
> after.  This may be impossible for script kiddies, but it won't be for
> nation states ... are you going to exclusively trust the random number
> you got from your chinese certified TPM?

I'd suggest approach where TPM RNG result is xored with krng result.

> Remember also that the attack doesn't have to be to the TPM only, it
> could be the pathway by which we get the random number, which involves
> components outside of the TPM certification.

Yeah, I do get this.

/Jarkko