[PATCH v2] tpm: display warning if using gpio reset with TPM

[Tim Harvey <tharvey@gateworks.com>]

Instead of displaying what looks like an error message if a
gpio-reset dt prop is missing for a TPM display a warning that
having a gpio reset on a TPM should not be used for a secure production
device.

TCG TIS spec [1] says:
"The TPM_Init (LRESET#/SPI_RST#) signal MUST be connected to the
platform CPU Reset signal such that it complies with the requirements
specified in section 1.2.7 HOST Platform Reset in the PC Client
Implementation Specification for Conventional BIOS."

The reasoning is that you should not be able to toggle a GPIO and reset
the TPM without resetting the CPU as well because if an attacker can
break into your OS via an OS level security flaw they can then reset the
TPM via GPIO and replay the measurements required to unseal keys
that you have otherwise protected.

[1] https://trustedcomputinggroup.org/wp-content/uploads/TCG_PCClientTPMInterfaceSpecification_TIS__1-3_27_03212013.pdf

Signed-off-by: Tim Harvey <tharvey@gateworks.com>
---
v2: change the message to a warning and update commit desc/log
---
 drivers/tpm/tpm2_tis_spi.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/tpm/tpm2_tis_spi.c b/drivers/tpm/tpm2_tis_spi.c
index de9cf8f21e07..c9c83f6f0fc8 100644
--- a/drivers/tpm/tpm2_tis_spi.c
+++ b/drivers/tpm/tpm2_tis_spi.c
@@ -237,14 +237,14 @@ static int tpm_tis_spi_probe(struct udevice *dev)
 			/* legacy reset */
 			ret = gpio_request_by_name(dev, "gpio-reset", 0,
 						   &reset_gpio, GPIOD_IS_OUT);
-			if (ret) {
-				log(LOGC_NONE, LOGL_NOTICE,
-				    "%s: missing reset GPIO\n", __func__);
+			if (ret)
 				goto init;
-			}
 			log(LOGC_NONE, LOGL_NOTICE,
 			    "%s: gpio-reset is deprecated\n", __func__);
 		}
+		log(LOGC_NONE, LOGL_WARNING,
+		    "%s: TPM gpio reset should not be used on secure production devices\n",
+		    dev->name);
 		dm_gpio_set_value(&reset_gpio, 1);
 		mdelay(1);
 		dm_gpio_set_value(&reset_gpio, 0);
-- 
2.25.1