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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Mon, 31 Jul 2023 12:30:56 -0700
Message-ID: <CAHk-=why64j-K4e1VxKwx7o6FiGjcXEnu1Pz+1QnNTBCv9AGyA@mail.gmail.com>
Subject: Re: [PATCH 1/1] tpm: disable hwrng for fTPM on some AMD designs
To:     "Limonciello, Mario" <mario.limonciello@amd.com>
Cc:     Jarkko Sakkinen <jarkko@kernel.org>,
        Daniil Stas <daniil.stas@posteo.net>,
        James.Bottomley@hansenpartnership.com, Jason@zx2c4.com,
        linux-integrity@vger.kernel.org, linux-kernel@vger.kernel.org,
        regressions@leemhuis.info, stable@vger.kernel.org
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On Mon, 31 Jul 2023 at 12:18, Limonciello, Mario
<mario.limonciello@amd.com> wrote:
>
> > Is there some way to just see "this is a fTPM"?
>
> How many fTPM implementations are there?  We're talking like less than 5
> right?  Maybe just check against a static list when
> calling tpm_add_hwrng().

Sounds sane. But I was hoping for some direct way to just query "are
you a firmware SMI hook, or real hardware".

It would be lovely to avoid the list, because maybe AMD does - or in
the past have done - discrete TPM hardware?  So it might not be as
easy as just checking against the manufacturer..

That said, maybe it really doesn't matter. I'm perfectly fine with
just the "check for AMD as a manufacturer" too.

In fact, I'd be perfectly happy with not using the TPM for run-time
randomness at all, and purely doing it for the bootup entropy, which
is where I feel it matters a lot m ore.

> I've had some discussions today with a variety of people on this problem
> and there is no advantage to get RNG through the fTPM over RDRAND.

Ack.

And that's true even if you _trust_ the fTPM.

That said, I see no real downside to using the TPM (whether firmware
or discrete) to just add to the boot-time "we'll gather entropy for
our random number generator from any source".

So it's purely the runtime randomness where I feel that the upside
just isn't there, and the downsides are real.

                  Linus