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

[Linus Torvalds]

On Mon, 31 Jul 2023 at 03:53, Jarkko Sakkinen <jarkko@xxxxxxxxxx> 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