On Mon Jul 31, 2023 at 10:05 PM EEST, Linus Torvalds wrote:

On Mon, 31 Jul 2023 at 03:53, Jarkko Sakkinen [off-list ref] wrote:

quoted

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

I would disable it inside tpm_crb driver, which is the driver used
for fTPM's: they are identified by MSFT0101 ACPI identifier.

I think the right scope is still AMD because we don't have such
regressions with Intel fTPM.

I.e. I would move the helper I created inside tpm_crb driver, and
a new flag, let's say "TPM_CHIP_FLAG_HWRNG_DISABLED", which tpm_crb
sets before calling tpm_chip_register().

Finally, tpm_add_hwrng() needs the following invariant:

	if (chip->flags & TPM_CHIP_FLAG_HWRNG_DISABLED)
		return 0;

How does this sound? I can refine this quickly from my first trial.

BR, Jarkko