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

Hi Miquel

On Mon, 8 Apr 2024 at 10:23, Miquel Raynal miquel.raynal@bootlin.com wrote:

Hello,

ilias.apalodimas@linaro.org wrote on Thu, 28 Mar 2024 09:08:37 +0200:

...

Thanks Tim

On Thu, 28 Mar 2024 at 00:12, Tim Harvey tharvey@gateworks.com wrote:

...

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_PCClientTPMInterfac...

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);
  

The current logic expects a reset gpio and bails out if it cannot get it with a (questionable) goto statement.

You want to invert that logic, and expect no gpio, but only if there is one you want to warn.

This is perfectly fine but the logic here must be clarified. I'd go for:

ret = gpio_request() if (ret) { ret = gpio_request() if (!ret) warn(deprecated) }

if (!ret) { warn(dangerous) toggle_value() }

I would ideally replace the 'if (ret)' clauses with 'if (!reset_gpio)' which would make the checks even clearer.

Fair enough. But the code with the proposed change has no functional problems right? If so I'll send a PR to Tom as is and rework it as suggested later

Thanks /Ilias

Thanks, Miquèl