Messages in this thread | | | Date | Tue, 01 Aug 2023 21:28:42 +0300 | From | "Jarkko Sakkinen" <> | Subject | Re: [PATCH 1/1] tpm: disable hwrng for fTPM on some AMD designs |
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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
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