On 7/31/2023 2:05 PM, Linus Torvalds wrote:
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"?
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
