tpm: disable hwrng for fTPM on some AMD designs

AMD has issued an advisory indicating that having fTPM enabled in
BIOS can cause "stuttering" in the OS.  This issue has been fixed
in newer versions of the fTPM firmware, but it's up to system
designers to decide whether to distribute it.

This issue has existed for a while, but is more prevalent starting
with kernel 6.1 because commit b006c439d58db ("hwrng: core - start
hwrng kthread also for untrusted sources") started to use the fTPM
for hwrng by default. However, all uses of /dev/hwrng result in
unacceptable stuttering.

So, simply disable registration of the defective hwrng when detecting
these faulty fTPM versions.  As this is caused by faulty firmware, it
is plausible that such a problem could also be reproduced by other TPM
interactions, but this hasn't been shown by any user's testing or reports.

It is hypothesized to be triggered more frequently by the use of the RNG
because userspace software will fetch random numbers regularly.

Intentionally continue to register other TPM functionality so that users
that rely upon PCR measurements or any storage of data will still have
access to it.  If it's found later that another TPM functionality is
exacerbating this problem a module parameter it can be turned off entirely
and a module parameter can be introduced to allow users who rely upon
fTPM functionality to turn it on even though this problem is present.

Link: https://www.amd.com/en/support/kb/faq/pa-410
Link: https://bugzilla.kernel.org/show_bug.cgi?id=216989
Link: https://lore.kernel.org/all/20230209153120.261904-1-Jason@zx2c4.com/
Fixes: b006c439d58d ("hwrng: core - start hwrng kthread also for untrusted sources")
Cc: stable@vger.kernel.org
Cc: Jarkko Sakkinen <jarkko@kernel.org>
Cc: Thorsten Leemhuis <regressions@leemhuis.info>
Cc: James Bottomley <James.Bottomley@hansenpartnership.com>
Tested-by: reach622@mailcuk.com
Tested-by: Bell <1138267643@qq.com>
Co-developed-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>

0 files changed, 0 insertions, 0 deletions