[PATCH v3] tpm: display warning if using gpio reset with TPM

[Jorge Ramirez-Ortiz, Foundries]

On 16/05/24 09:09:50, Miquel Raynal wrote:
> Hi Tim,
> 
> tharvey at gateworks.com wrote on Wed, 15 May 2024 16:21:38 -0700:
> 
> > 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.
> > 
> > Additionally restructure the code for improved readability allowing for
> > removal of the init label.
> > 
> > Before:
> >  - board with no reset gpio
> > u-boot=> tpm init && tpm info
> > tpm_tis_spi_probe: missing reset GPIO
> > tpm at 1 v2.0: VendorID 0x1114, DeviceID 0x3205, RevisionID 0x01 [open]
> >  - board with a reset gpio
> > u-boot=> tpm init && tpm info
> > tpm at 1 v2.0: VendorID 0x1114, DeviceID 0x3205, RevisionID 0x01 [open]
> > 
> > After:
> >  - board with no reset gpio
> > u-boot=> tpm init && tpm info
> > tpm at 1 v2.0: VendorID 0x1114, DeviceID 0x3205, RevisionID 0x01 [open]
> >  - board with a reset gpio
> > u-boot=> tpm init && tpm info
> > tpm at 1: TPM gpio reset should not be used on secure production devices
> > tpm at 1 v2.0: VendorID 0x1114, DeviceID 0x3205, RevisionID 0x01 [open]
> > 
> > [1] https://trustedcomputinggroup.org/wp-content/uploads/TCG_PCClientTPMInterfaceSpecification_TIS__1-3_27_03212013.pdf
> > 
> > Signed-off-by: Tim Harvey <tharvey at gateworks.com>
> 
> Looks way cleaner, thanks.
> 
> Reviewed-by: Miquel Raynal <miquel.raynal at bootlin.com>
> 
> Miquèl

nice. if needed

Signed-off-by: Jorge Ramirez-Ortiz <jorge at foundries.io>