linux-kernel - [PATCH v3] tpm: Enable hwrng only for Pluton on AMD CPUs

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Date:   Wed, 23 Aug 2023 02:15:10 +0300
From:   Jarkko Sakkinen <jarkko@...nel.org>
To:     linux-integrity@...r.kernel.org
Cc:     Jerry Snitselaar <jsnitsel@...hat.com>,
        Jarkko Sakkinen <jarkko@...nel.org>, stable@...r.kernel.org,
        Todd Brandt <todd.e.brandt@...el.com>,
        Peter Huewe <peterhuewe@....de>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Mario Limonciello <mario.limonciello@....com>,
        linux-kernel@...r.kernel.org
Subject: [PATCH v3] tpm: Enable hwrng only for Pluton on AMD CPUs

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@...r.kernel.org
Fixes: 554b841d4703 ("tpm: Disable RNG for all AMD fTPMs")
Reported-by: Todd Brandt <todd.e.brandt@...el.com>
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=217804
Signed-off-by: Jarkko Sakkinen <jarkko@...nel.org>
---
v3:
* Forgot to amend config flags.
v2:
* CONFIG_X86
* Removed "Reviewed-by: Jarkko Sakkinen <jarkko@...nel.org>"
* Removed "Signed-off-by: Mario Limonciello <mario.limonciello@....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");
+		chip->flags |= TPM_CHIP_FLAG_HWRNG_DISABLED;
+	}
+#endif /* CONFIG_X86 */
 
 	rc = tpm_chip_register(chip);
 
-- 
2.39.2