On 7/31/2023 2:05 PM, 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"?
How many fTPM implementations are there? We're talking like less than 5 right? Maybe just check against a static list when calling tpm_add_hwrng().
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.
I've had some discussions today with a variety of people on this problem and there is no advantage to get RNG through the fTPM over RDRAND.
They both source the exact same hardware IP, but RDRAND is a *lot* more direct.
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