linux-kernel - Re: [PATCH 1/1] tpm: disable hwrng for fTPM on some AMD designs

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Date:   Mon, 31 Jul 2023 12:05:41 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Jarkko Sakkinen <jarkko@...nel.org>
Cc:     Daniil Stas <daniil.stas@...teo.net>,
        Mario Limonciello <mario.limonciello@....com>,
        James.Bottomley@...senpartnership.com, Jason@...c4.com,
        linux-integrity@...r.kernel.org, linux-kernel@...r.kernel.org,
        regressions@...mhuis.info, stable@...r.kernel.org
Subject: Re: [PATCH 1/1] tpm: disable hwrng for fTPM on some AMD designs

On Mon, 31 Jul 2023 at 03:53, Jarkko Sakkinen <jarkko@...nel.org> wrote:
>
> I quickly carved up a patch (attached), which is only compile tested
> because I do not have any AMD hardware at hand.

Is there some way to just see "this is a fTPM"?

Because honestly, even if AMD is the one that has had stuttering
issues, the bigger argument is that there is simply no _point_ in
supporting randomness from a firmware source.

There is no way anybody should believe that a firmware TPM generates
better randomness than we do natively.

And there are many reasons to _not_ believe it. The AMD problem is
just the most user-visible one.

Now, I'm not saying that a fTPM needs to be disabled in general - but
I really feel like we should just do

 static int tpm_add_hwrng(struct tpm_chip *chip)
 {
        if (!IS_ENABLED(CONFIG_HW_RANDOM_TPM))
                return 0;
        // If it's not hardware, don't treat it as such
        if (tpm_is_fTPM(chip))
                return 0;
        [...]

and be done with it.

But hey, if we have no way to see that whole "this is firmware
emulation", then just blocking AMD might be the only way.

               Linus