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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Mon, 31 Jul 2023 12:05:41 -0700
Message-ID: <CAHk-=whphk8Jp=NYmnm7Qv+vZ6ScYCz+rV8a2G1nD-AQY3z+mQ@mail.gmail.com>
Subject: Re: [PATCH 1/1] tpm: disable hwrng for fTPM on some AMD designs
To:     Jarkko Sakkinen <jarkko@kernel.org>
Cc:     Daniil Stas <daniil.stas@posteo.net>,
        Mario Limonciello <mario.limonciello@amd.com>,
        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 03:53, Jarkko Sakkinen <jarkko@kernel.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