[PATCH v2 1/2] tpm: Disable a TPM that stops responding

[Breno Leitao <leitao@debian.org>]

A TPM that stops responding fails each command only after the transport
timeout, and its callers keep sending more: the hwrng thread every 10
seconds forever, and userspace through /dev/tpmrm0. Each attempt holds
tpm_mutex for the whole timeout and logs an error:

  tpm tpm0: invalid TPM_STS.x 0xff, dumping stack for forensics
  tpm tpm0: tpm_try_transmit: send(): error -62
  tpm tpm0: tpm_try_transmit: send(): error -62
  ...

The core already stops using a TPM it cannot trust: tpm_try_get_ops()
fails with -EIO once TPM_CHIP_FLAG_DISABLE is set, which
tpm2_load_null() does when it cannot restore the null key. Set the same
flag once 60 commands in a row have timed out.

A completed command, whatever its TPM return code, resets the count.
Some TPMs stall for half a minute, time out a handful of commands, and
then recover, so a short streak must not disable them.

With only the hwrng retrying every 10 seconds, 60 timeouts take more
than ten minutes. A userspace poller gets there sooner, but each of its
commands still waits out the transport timeout.

The limit is the tpm.max_unanswered boot parameter, so that slow
pre-production parts, such as FPGA prototypes, can raise it, or set it
to 0 to never disable the TPM. A disabled TPM stays disabled until the
driver is rebound or the machine reboots.

Signed-off-by: Breno Leitao <leitao@debian.org>
---
 drivers/char/tpm/tpm-interface.c | 15 +++++++++++++++
 include/linux/tpm.h              |  1 +
 2 files changed, 16 insertions(+)

diff --git a/drivers/char/tpm/tpm-interface.c b/drivers/char/tpm/tpm-interface.c
index e0385b16616b46..b7865ccb799289 100644
--- a/drivers/char/tpm/tpm-interface.c
+++ b/drivers/char/tpm/tpm-interface.c
@@ -40,6 +40,11 @@ module_param_named(suspend_pcr, tpm_suspend_pcr, uint, 0644);
 MODULE_PARM_DESC(suspend_pcr,
 		 "PCR to use for dummy writes to facilitate flush on suspend.");
 
+static unsigned int tpm_max_unanswered = 60;
+module_param_named(max_unanswered, tpm_max_unanswered, uint, 0444);
+MODULE_PARM_DESC(max_unanswered,
+		 "Disable the TPM after this many commands in a row time out (0 = never).");
+
 /**
  * tpm_calc_ordinal_duration() - calculate the maximum command duration
  * @chip:    TPM chip to use.
@@ -268,6 +273,16 @@ ssize_t tpm_transmit(struct tpm_chip *chip, u8 *buf, size_t bufsiz)
 		delay_msec *= 2;
 		memcpy(buf, save, save_size);
 	}
+
+	if (ret >= 0) {
+		/* we are good, reset nr_unanswered */
+		chip->nr_unanswered = 0;
+	} else if (ret == -ETIME &&
+		   ++chip->nr_unanswered == tpm_max_unanswered) {
+		dev_err(&chip->dev, "%u commands in a row timed out, disabling the TPM\n",
+			chip->nr_unanswered);
+		chip->flags |= TPM_CHIP_FLAG_DISABLE;
+	}
 	return ret;
 }
 
diff --git a/include/linux/tpm.h b/include/linux/tpm.h
index 0db277af45c393..e8191585a68e7a 100644
--- a/include/linux/tpm.h
+++ b/include/linux/tpm.h
@@ -131,6 +131,7 @@ struct tpm_chip {
 	struct hwrng hwrng;
 
 	struct mutex tpm_mutex;	/* tpm is processing */
+	unsigned int nr_unanswered;	/* commands in a row that timed out */
 
 	unsigned long timeout_a; /* jiffies */
 	unsigned long timeout_b; /* jiffies */

-- 
2.53.0-Meta