Re: [PATCH v3] tpm: Enable hwrng only for Pluton on AMD CPUs

From: Paul Menzel <pmenzel@molgen.mpg.de>
To: Jarkko Sakkinen <jarkko@kernel.org>
Cc: linux-integrity@vger.kernel.org,
	Jerry Snitselaar <jsnitsel@redhat.com>,
	stable@vger.kernel.org, Todd Brandt <todd.e.brandt@intel.com>,
	Peter Huewe <peterhuewe@gmx.de>, Jason Gunthorpe <jgg@ziepe.ca>,
	Mario Limonciello <mario.limonciello@amd.com>,
	linux-kernel@vger.kernel.org, Patrick Steinhardt <ps@pks.im>,
	Ronan Pigott <ronan@rjp.ie>, Raymond Jay Golo <rjgolo@gmail.com>
Subject: Re: [PATCH v3] tpm: Enable hwrng only for Pluton on AMD CPUs
Date: Wed, 23 Aug 2023 10:23:13 +0200	[thread overview]
Message-ID: <705b9769-4132-450b-bd47-2423c419db2a@molgen.mpg.de> (raw)
In-Reply-To: <20230822231510.2263255-1-jarkko@kernel.org>

Dear Jarkko,

Thank you for your patch.

Am 23.08.23 um 01:15 schrieb Jarkko Sakkinen:
> The vendor check introduced by commit 554b841d4703 ("tpm: Disable RNG for
> all AMD fTPMs") doesn't work properly on a number of Intel fTPMs.  On the
> reported systems the TPM doesn't reply at bootup and returns back the
> command code. This makes the TPM fail probe.
> 
> Since only Microsoft Pluton is the only known combination of AMD CPU and
> fTPM from other vendor, disable hwrng otherwise. In order to make sysadmin
> aware of this, print also info message to the klog.
> 
> Cc: stable@vger.kernel.org
> Fixes: 554b841d4703 ("tpm: Disable RNG for all AMD fTPMs")
> Reported-by: Todd Brandt <todd.e.brandt@intel.com>
> Closes: https://bugzilla.kernel.org/show_bug.cgi?id=217804
> Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>

Mario’s patch also had the three reporters below listed:

Reported-by: Patrick Steinhardt <ps@pks.im>
Reported-by: Ronan Pigott <ronan@rjp.ie>
Reported-by: Raymond Jay Golo <rjgolo@gmail.com>

> ---
> v3:
> * Forgot to amend config flags.
> v2:
> * CONFIG_X86
> * Removed "Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>"
> * Removed "Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>"
> ---
>   drivers/char/tpm/tpm_crb.c | 33 ++++++++-------------------------
>   1 file changed, 8 insertions(+), 25 deletions(-)
> 
> diff --git a/drivers/char/tpm/tpm_crb.c b/drivers/char/tpm/tpm_crb.c
> index 65ff4d2fbe8d..ea085b14ab7c 100644
> --- a/drivers/char/tpm/tpm_crb.c
> +++ b/drivers/char/tpm/tpm_crb.c
> @@ -463,28 +463,6 @@ static bool crb_req_canceled(struct tpm_chip *chip, u8 status)
>   	return (cancel & CRB_CANCEL_INVOKE) == CRB_CANCEL_INVOKE;
>   }
>   
> -static int crb_check_flags(struct tpm_chip *chip)
> -{
> -	u32 val;
> -	int ret;
> -
> -	ret = crb_request_locality(chip, 0);
> -	if (ret)
> -		return ret;
> -
> -	ret = tpm2_get_tpm_pt(chip, TPM2_PT_MANUFACTURER, &val, NULL);
> -	if (ret)
> -		goto release;
> -
> -	if (val == 0x414D4400U /* AMD */)
> -		chip->flags |= TPM_CHIP_FLAG_HWRNG_DISABLED;
> -
> -release:
> -	crb_relinquish_locality(chip, 0);
> -
> -	return ret;
> -}
> -
>   static const struct tpm_class_ops tpm_crb = {
>   	.flags = TPM_OPS_AUTO_STARTUP,
>   	.status = crb_status,
> @@ -827,9 +805,14 @@ static int crb_acpi_add(struct acpi_device *device)
>   	if (rc)
>   		goto out;
>   
> -	rc = crb_check_flags(chip);
> -	if (rc)
> -		goto out;
> +#ifdef CONFIG_X86
> +	/* A quirk for https://www.amd.com/en/support/kb/faq/pa-410 */
> +	if (boot_cpu_data.x86_vendor == X86_VENDOR_AMD &&
> +	    priv->sm != ACPI_TPM2_COMMAND_BUFFER_WITH_PLUTON) {
> +		dev_info(dev, "Disabling hwrng\n");

A more elaborate log message would be helpful for the user. Maybe:

Disabling hwrng in AMD's fTPM to avoid stutter (AMD article PA 410)

> +		chip->flags |= TPM_CHIP_FLAG_HWRNG_DISABLED;
> +	}
> +#endif /* CONFIG_X86 */
>   
>   	rc = tpm_chip_register(chip);
>   

Kind regards,

Paul