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