Re: [PATCH v3 01/11] tpm: Add support for in-kernel resetting of PCRs

[Jarkko Sakkinen <jarkko@kernel.org>]

On Tue, Sep 27, 2022 at 09:49:12AM -0700, Evan Green wrote:
> From: Matthew Garrett <matthewgarrett@google.com>
> 
> Add an internal command for resetting a PCR. This will be used by the
> encrypted hibernation code to set PCR23 to a known value. The
> hibernation code will seal the hibernation key with a policy specifying
> PCR23 be set to this known value as a mechanism to ensure that the
> hibernation key is genuine. But to do this repeatedly, resetting the PCR
> is necessary as well.
> 
> Link: https://lore.kernel.org/lkml/20210220013255.1083202-2-matthewgarrett@google.com/
> Signed-off-by: Matthew Garrett <mjg59@google.com>
> Signed-off-by: Evan Green <evgreen@chromium.org>
> ---
> 
> Changes in v3:
>  - Unify tpm1/2_pcr_reset prototypes (Jarkko)
>  - Wait no, remove the TPM1 stuff altogether (Jarkko)
>  - Remove extra From tag and blank in commit msg (Jarkko).
> 
>  drivers/char/tpm/tpm-interface.c | 25 ++++++++++++++++++++++
>  drivers/char/tpm/tpm.h           |  1 +
>  drivers/char/tpm/tpm2-cmd.c      | 36 ++++++++++++++++++++++++++++++++
>  include/linux/tpm.h              |  7 +++++++
>  4 files changed, 69 insertions(+)
> 
> diff --git a/drivers/char/tpm/tpm-interface.c b/drivers/char/tpm/tpm-interface.c
> index 1621ce8187052c..2ac9079860b1e0 100644
> --- a/drivers/char/tpm/tpm-interface.c
> +++ b/drivers/char/tpm/tpm-interface.c
> @@ -342,6 +342,31 @@ int tpm_pcr_extend(struct tpm_chip *chip, u32 pcr_idx,
>  }
>  EXPORT_SYMBOL_GPL(tpm_pcr_extend);
>  
> +/**
> + * tpm_pcr_reset - reset the specified PCR
> + * @chip:	a &struct tpm_chip instance, %NULL for the default chip
> + * @pcr_idx:	the PCR to be reset
> + *
> + * Return: same as with tpm_transmit_cmd(), or ENOTTY for TPM1 devices.
> + */
> +int tpm_pcr_reset(struct tpm_chip *chip, u32 pcr_idx)
> +{
> +	int rc;
> +
> +	chip = tpm_find_get_ops(chip);
> +	if (!chip)
> +		return -ENODEV;
> +
> +	if (chip->flags & TPM_CHIP_FLAG_TPM2)
> +		rc = tpm2_pcr_reset(chip, pcr_idx);
> +	else
> +		rc = -ENOTTY;
> +
> +	tpm_put_ops(chip);
> +	return rc;
> +}
> +EXPORT_SYMBOL_GPL(tpm_pcr_reset);

Please rename this as tpm2_pcr_reset() and open code the contents
of the current tpm2_pcd_reset() into it, as this layering is now
obsolete.

> +
>  /**
>   * tpm_send - send a TPM command
>   * @chip:	a &struct tpm_chip instance, %NULL for the default chip
> diff --git a/drivers/char/tpm/tpm.h b/drivers/char/tpm/tpm.h
> index 24ee4e1cc452a0..34e20b3192f833 100644
> --- a/drivers/char/tpm/tpm.h
> +++ b/drivers/char/tpm/tpm.h
> @@ -217,6 +217,7 @@ int tpm2_pcr_read(struct tpm_chip *chip, u32 pcr_idx,
>  		  struct tpm_digest *digest, u16 *digest_size_ptr);
>  int tpm2_pcr_extend(struct tpm_chip *chip, u32 pcr_idx,
>  		    struct tpm_digest *digests);
> +int tpm2_pcr_reset(struct tpm_chip *chip, u32 pcr_idx);
>  int tpm2_get_random(struct tpm_chip *chip, u8 *dest, size_t max);
>  ssize_t tpm2_get_tpm_pt(struct tpm_chip *chip, u32 property_id,
>  			u32 *value, const char *desc);
> diff --git a/drivers/char/tpm/tpm2-cmd.c b/drivers/char/tpm/tpm2-cmd.c
> index 65d03867e114c5..69126a6770386e 100644
> --- a/drivers/char/tpm/tpm2-cmd.c
> +++ b/drivers/char/tpm/tpm2-cmd.c
> @@ -269,6 +269,42 @@ int tpm2_pcr_extend(struct tpm_chip *chip, u32 pcr_idx,
>  	return rc;
>  }
>  
> +/**
> + * tpm2_pcr_reset() - reset a PCR
> + *

Remove empty line and "Reset a PCR" with a capital.

There might be random conventions in this but I'm now referring
to just what this says:

https://www.kernel.org/doc/html/v5.19/doc-guide/kernel-doc.html

I.e. better to align with documentation for this and future patches.

> + * @chip:	TPM chip to use.
> + * @pcr_idx:	index of the PCR.
> + *
> + * Return: Same as with tpm_transmit_cmd.
> + */
> +int tpm2_pcr_reset(struct tpm_chip *chip, u32 pcr_idx)
> +{
> +	struct tpm_buf buf;
> +	struct tpm2_null_auth_area auth_area;
> +	int rc;
> +
> +	rc = tpm_buf_init(&buf, TPM2_ST_SESSIONS, TPM2_CC_PCR_RESET);
> +	if (rc)
> +		return rc;
> +
> +	tpm_buf_append_u32(&buf, pcr_idx);
> +
> +	auth_area.handle = cpu_to_be32(TPM2_RS_PW);
> +	auth_area.nonce_size = 0;
> +	auth_area.attributes = 0;
> +	auth_area.auth_size = 0;
> +
> +	tpm_buf_append_u32(&buf, sizeof(struct tpm2_null_auth_area));
> +	tpm_buf_append(&buf, (const unsigned char *)&auth_area,
> +		       sizeof(auth_area));
> +
> +	rc = tpm_transmit_cmd(chip, &buf, 0, "attempting to reset a PCR");
> +
> +	tpm_buf_destroy(&buf);
> +
> +	return rc;
> +}
> +
>  struct tpm2_get_random_out {
>  	__be16 size;
>  	u8 buffer[TPM_MAX_RNG_DATA];
> diff --git a/include/linux/tpm.h b/include/linux/tpm.h
> index dfeb25a0362dee..8320cbac6f4009 100644
> --- a/include/linux/tpm.h
> +++ b/include/linux/tpm.h
> @@ -219,6 +219,7 @@ enum tpm2_command_codes {
>  	TPM2_CC_HIERARCHY_CONTROL       = 0x0121,
>  	TPM2_CC_HIERARCHY_CHANGE_AUTH   = 0x0129,
>  	TPM2_CC_CREATE_PRIMARY          = 0x0131,
> +	TPM2_CC_PCR_RESET		= 0x013D,
>  	TPM2_CC_SEQUENCE_COMPLETE       = 0x013E,
>  	TPM2_CC_SELF_TEST	        = 0x0143,
>  	TPM2_CC_STARTUP		        = 0x0144,
> @@ -423,6 +424,7 @@ extern ssize_t tpm_transmit_cmd(struct tpm_chip *chip, struct tpm_buf *buf,
>  				size_t min_rsp_body_length, const char *desc);
>  extern int tpm_pcr_read(struct tpm_chip *chip, u32 pcr_idx,
>  			struct tpm_digest *digest);
> +extern int tpm_pcr_reset(struct tpm_chip *chip, u32 pcr_idx);
>  extern int tpm_pcr_extend(struct tpm_chip *chip, u32 pcr_idx,
>  			  struct tpm_digest *digests);
>  extern int tpm_send(struct tpm_chip *chip, void *cmd, size_t buflen);
> @@ -440,6 +442,11 @@ static inline int tpm_pcr_read(struct tpm_chip *chip, int pcr_idx,
>  	return -ENODEV;
>  }
>  
> +static inline int tpm_pcr_reset(struct tpm_chip *chip, int pcr_idx)
> +{
> +	return -ENODEV;
> +}
> +
>  static inline int tpm_pcr_extend(struct tpm_chip *chip, u32 pcr_idx,
>  				 struct tpm_digest *digests)
>  {
> -- 
> 2.31.0
> 

BR, Jarkko