Re: [PATCH v4 09/13] tpm: add hmac checks to tpm2_pcr_extend()

[Jarkko Sakkinen <jarkko@kernel.org>]

On Mon, 2023-04-03 at 17:39 -0400, James Bottomley wrote:
> tpm2_pcr_extend() is used by trusted keys to extend a PCR to prevent a
> key from being re-loaded until the next reboot.  To use this
> functionality securely, that extend must be protected by a session
> hmac.  This patch adds HMAC protection so tampering with the
> tpm2_pcr_extend() command in flight is detected.
> 
> Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>

What the heck is "check"?

The code change adds hmac pipeline for the command.

I get the code change but the description is misleading as this
does more than just add a check.

> ---
>  drivers/char/tpm/tpm2-cmd.c | 27 ++++++++++-----------------
>  1 file changed, 10 insertions(+), 17 deletions(-)
> 
> diff --git a/drivers/char/tpm/tpm2-cmd.c b/drivers/char/tpm/tpm2-cmd.c
> index b0e72fb563d9..a53a843294ed 100644
> --- a/drivers/char/tpm/tpm2-cmd.c
> +++ b/drivers/char/tpm/tpm2-cmd.c
> @@ -216,13 +216,6 @@ int tpm2_pcr_read(struct tpm_chip *chip, u32 pcr_idx,
>  	return rc;
>  }
>  
> -struct tpm2_null_auth_area {
> -	__be32  handle;
> -	__be16  nonce_size;
> -	u8  attributes;
> -	__be16  auth_size;
> -} __packed;
> -
>  /**
>   * tpm2_pcr_extend() - extend a PCR value
>   *
> @@ -236,24 +229,22 @@ int tpm2_pcr_extend(struct tpm_chip *chip, u32 pcr_idx,
>  		    struct tpm_digest *digests)
>  {
>  	struct tpm_buf buf;
> -	struct tpm2_null_auth_area auth_area;
>  	int rc;
>  	int i;
>  
> -	rc = tpm_buf_init(&buf, TPM2_ST_SESSIONS, TPM2_CC_PCR_EXTEND);
> +	rc = tpm2_start_auth_session(chip);
>  	if (rc)
>  		return rc;
>  
> -	tpm_buf_append_u32(&buf, pcr_idx);
> +	rc = tpm_buf_init(&buf, TPM2_ST_SESSIONS, TPM2_CC_PCR_EXTEND);
> +	if (rc) {
> +		tpm2_end_auth_session(chip);
> +		return rc;
> +	}
>  
> -	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_name(chip, &buf, pcr_idx, NULL);
> +	tpm_buf_append_hmac_session(chip, &buf, 0, NULL, 0);
>  
> -	tpm_buf_append_u32(&buf, sizeof(struct tpm2_null_auth_area));
> -	tpm_buf_append(&buf, (const unsigned char *)&auth_area,
> -		       sizeof(auth_area));
>  	tpm_buf_append_u32(&buf, chip->nr_allocated_banks);
>  
>  	for (i = 0; i < chip->nr_allocated_banks; i++) {
> @@ -262,7 +253,9 @@ int tpm2_pcr_extend(struct tpm_chip *chip, u32 pcr_idx,
>  			       chip->allocated_banks[i].digest_size);
>  	}
>  
> +	tpm_buf_fill_hmac_session(chip, &buf);
>  	rc = tpm_transmit_cmd(chip, &buf, 0, "attempting extend a PCR value");
> +	rc = tpm_buf_check_hmac_response(chip, &buf, rc);
>  
>  	tpm_buf_destroy(&buf);
>  

BR, Jarkko