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

[Miquel Raynal]

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