[PATCH v2] tpm: disable hwrng for fTPM on some AMD designs

Mario Limonciello
2023-02-20T18:08:04Z

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>
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>
---
v1->v2:
* Minor style from Jarkko's feedback
* Move comment above function
* Explain further in commit message
---
drivers/char/tpm/tpm-chip.c | 61 ++++++++++++++++++++++++++++++-
drivers/char/tpm/tpm.h | 73 +++++++++++++++++++++++++++++++++++++
2 files changed, 133 insertions(+), 1 deletion(-)

diff --git a/drivers/char/tpm/tpm-chip.c b/drivers/char/tpm/tpm-chip.c
index 741d8f3e8fb3..1b066d7a6e21 100644
--- a/drivers/char/tpm/tpm-chip.c
+++ b/drivers/char/tpm/tpm-chip.c
@@ -512,6 +512,64 @@ static int tpm_add_legacy_sysfs(struct tpm_chip *chip)
return 0;
}

+/*
+ * Some AMD fTPM versions may cause stutter
+ * https://www.amd.com/en/support/kb/faq/pa-410
+ *
+ * Fixes are available in two series of fTPM firmware:
+ * 6.x.y.z series: 6.0.18.6 +
+ * 3.x.y.z series: 3.57.y.5 +
+ */
+static bool tpm_amd_is_rng_defective(struct tpm_chip *chip)
+{
+ u32 val1, val2;
+ u64 version;
+ int ret;
+
+ if (!(chip->flags & TPM_CHIP_FLAG_TPM2))
+ return false;
+
+ ret = tpm_request_locality(chip);
+ if (ret)
+ return false;
+
+ ret = tpm2_get_tpm_pt(chip, TPM2_PT_MANUFACTURER, &val1, NULL);
+ if (ret)
+ goto release;
+ if (val1 != 0x414D4400U /* AMD */) {
+ ret = -ENODEV;
+ goto release;
+ }
+ ret = tpm2_get_tpm_pt(chip, TPM2_PT_FIRMWARE_VERSION_1, &val1, NULL);
+ if (ret)
+ goto release;
+ ret = tpm2_get_tpm_pt(chip, TPM2_PT_FIRMWARE_VERSION_2, &val2, NULL);
+ if (ret)
+ goto release;
+
+release:
+ tpm_relinquish_locality(chip);
+
+ if (ret)
+ return false;
+
+ version = ((u64)val1 << 32) | val2;
+ if ((version >> 48) == 6) {
+ if (version >= 0x0006000000180006ULL)
+ return false;
+ } else if ((version >> 48) == 3) {
+ if (version >= 0x0003005700000005ULL)
+ return false;
+ } else
+ return false;
+
+ dev_warn(&chip->dev,
+ "AMD fTPM version 0x%llx causes system stutter; hwrng disabled\n",
+ version);
+
+ return true;
+}
+
static int tpm_hwrng_read(struct hwrng *rng, void *data, size_t max, bool wait)
{
struct tpm_chip *chip = container_of(rng, struct tpm_chip, hwrng);
@@ -521,7 +579,8 @@ static int tpm_hwrng_read(struct hwrng *rng, void *data, size_t max, bool wait)

static int tpm_add_hwrng(struct tpm_chip *chip)
{
- if (!IS_ENABLED(CONFIG_HW_RANDOM_TPM) || tpm_is_firmware_upgrade(chip))
+ if (!IS_ENABLED(CONFIG_HW_RANDOM_TPM) || tpm_is_firmware_upgrade(chip) ||
+ tpm_amd_is_rng_defective(chip))
return 0;

snprintf(chip->hwrng_name, sizeof(chip->hwrng_name),
diff --git a/drivers/char/tpm/tpm.h b/drivers/char/tpm/tpm.h
index 24ee4e1cc452..830014a26609 100644
--- a/drivers/char/tpm/tpm.h
+++ b/drivers/char/tpm/tpm.h
@@ -150,6 +150,79 @@ enum tpm_sub_capabilities {
TPM_CAP_PROP_TIS_DURATION = 0x120,
};

+enum tpm2_pt_props {
+ TPM2_PT_NONE = 0x00000000,
+ TPM2_PT_GROUP = 0x00000100,
+ TPM2_PT_FIXED = TPM2_PT_GROUP * 1,
+ TPM2_PT_FAMILY_INDICATOR = TPM2_PT_FIXED + 0,
+ TPM2_PT_LEVEL = TPM2_PT_FIXED + 1,
+ TPM2_PT_REVISION = TPM2_PT_FIXED + 2,
+ TPM2_PT_DAY_OF_YEAR = TPM2_PT_FIXED + 3,
+ TPM2_PT_YEAR = TPM2_PT_FIXED + 4,
+ TPM2_PT_MANUFACTURER = TPM2_PT_FIXED + 5,
+ TPM2_PT_VENDOR_STRING_1 = TPM2_PT_FIXED + 6,
+ TPM2_PT_VENDOR_STRING_2 = TPM2_PT_FIXED + 7,
+ TPM2_PT_VENDOR_STRING_3 = TPM2_PT_FIXED + 8,
+ TPM2_PT_VENDOR_STRING_4 = TPM2_PT_FIXED + 9,
+ TPM2_PT_VENDOR_TPM_TYPE = TPM2_PT_FIXED + 10,
+ TPM2_PT_FIRMWARE_VERSION_1 = TPM2_PT_FIXED + 11,
+ TPM2_PT_FIRMWARE_VERSION_2 = TPM2_PT_FIXED + 12,
+ TPM2_PT_INPUT_BUFFER = TPM2_PT_FIXED + 13,
+ TPM2_PT_HR_TRANSIENT_MIN = TPM2_PT_FIXED + 14,
+ TPM2_PT_HR_PERSISTENT_MIN = TPM2_PT_FIXED + 15,
+ TPM2_PT_HR_LOADED_MIN = TPM2_PT_FIXED + 16,
+ TPM2_PT_ACTIVE_SESSIONS_MAX = TPM2_PT_FIXED + 17,
+ TPM2_PT_PCR_COUNT = TPM2_PT_FIXED + 18,
+ TPM2_PT_PCR_SELECT_MIN = TPM2_PT_FIXED + 19,
+ TPM2_PT_CONTEXT_GAP_MAX = TPM2_PT_FIXED + 20,
+ TPM2_PT_NV_COUNTERS_MAX = TPM2_PT_FIXED + 22,
+ TPM2_PT_NV_INDEX_MAX = TPM2_PT_FIXED + 23,
+ TPM2_PT_MEMORY = TPM2_PT_FIXED + 24,
+ TPM2_PT_CLOCK_UPDATE = TPM2_PT_FIXED + 25,
+ TPM2_PT_CONTEXT_HASH = TPM2_PT_FIXED + 26,
+ TPM2_PT_CONTEXT_SYM = TPM2_PT_FIXED + 27,
+ TPM2_PT_CONTEXT_SYM_SIZE = TPM2_PT_FIXED + 28,
+ TPM2_PT_ORDERLY_COUNT = TPM2_PT_FIXED + 29,
+ TPM2_PT_MAX_COMMAND_SIZE = TPM2_PT_FIXED + 30,
+ TPM2_PT_MAX_RESPONSE_SIZE = TPM2_PT_FIXED + 31,
+ TPM2_PT_MAX_DIGEST = TPM2_PT_FIXED + 32,
+ TPM2_PT_MAX_OBJECT_CONTEXT = TPM2_PT_FIXED + 33,
+ TPM2_PT_MAX_SESSION_CONTEXT = TPM2_PT_FIXED + 34,
+ TPM2_PT_PS_FAMILY_INDICATOR = TPM2_PT_FIXED + 35,
+ TPM2_PT_PS_LEVEL = TPM2_PT_FIXED + 36,
+ TPM2_PT_PS_REVISION = TPM2_PT_FIXED + 37,
+ TPM2_PT_PS_DAY_OF_YEAR = TPM2_PT_FIXED + 38,
+ TPM2_PT_PS_YEAR = TPM2_PT_FIXED + 39,
+ TPM2_PT_SPLIT_MAX = TPM2_PT_FIXED + 40,
+ TPM2_PT_TOTAL_COMMANDS = TPM2_PT_FIXED + 41,
+ TPM2_PT_LIBRARY_COMMANDS = TPM2_PT_FIXED + 42,
+ TPM2_PT_VENDOR_COMMANDS = TPM2_PT_FIXED + 43,
+ TPM2_PT_NV_BUFFER_MAX = TPM2_PT_FIXED + 44,
+ TPM2_PT_MODES = TPM2_PT_FIXED + 45,
+ TPM2_PT_MAX_CAP_BUFFER = TPM2_PT_FIXED + 46,
+ TPM2_PT_VAR = TPM2_PT_GROUP * 2,
+ TPM2_PT_PERMANENT = TPM2_PT_VAR + 0,
+ TPM2_PT_STARTUP_CLEAR = TPM2_PT_VAR + 1,
+ TPM2_PT_HR_NV_INDEX = TPM2_PT_VAR + 2,
+ TPM2_PT_HR_LOADED = TPM2_PT_VAR + 3,
+ TPM2_PT_HR_LOADED_AVAIL = TPM2_PT_VAR + 4,
+ TPM2_PT_HR_ACTIVE = TPM2_PT_VAR + 5,
+ TPM2_PT_HR_ACTIVE_AVAIL = TPM2_PT_VAR + 6,
+ TPM2_PT_HR_TRANSIENT_AVAIL = TPM2_PT_VAR + 7,
+ TPM2_PT_HR_PERSISTENT = TPM2_PT_VAR + 8,
+ TPM2_PT_HR_PERSISTENT_AVAIL = TPM2_PT_VAR + 9,
+ TPM2_PT_NV_COUNTERS = TPM2_PT_VAR + 10,
+ TPM2_PT_NV_COUNTERS_AVAIL = TPM2_PT_VAR + 11,
+ TPM2_PT_ALGORITHM_SET = TPM2_PT_VAR + 12,
+ TPM2_PT_LOADED_CURVES = TPM2_PT_VAR + 13,
+ TPM2_PT_LOCKOUT_COUNTER = TPM2_PT_VAR + 14,
+ TPM2_PT_MAX_AUTH_FAIL = TPM2_PT_VAR + 15,
+ TPM2_PT_LOCKOUT_INTERVAL = TPM2_PT_VAR + 16,
+ TPM2_PT_LOCKOUT_RECOVERY = TPM2_PT_VAR + 17,
+ TPM2_PT_NV_WRITE_RECOVERY = TPM2_PT_VAR + 18,
+ TPM2_PT_AUDIT_COUNTER_0 = TPM2_PT_VAR + 19,
+ TPM2_PT_AUDIT_COUNTER_1 = TPM2_PT_VAR + 20,
+};

/* 128 bytes is an arbitrary cap. This could be as large as TPM_BUFSIZE - 18
* bytes, but 128 is still a relatively large number of random bytes and
--
2.34.1