linux-kernel - Re: [PATCH v5 01/11] tpm: Add support for in-kernel resetting of PCRs

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Date:   Sun, 13 Nov 2022 12:31:23 -0800
From:   Eric Biggers <ebiggers@...nel.org>
To:     Evan Green <evgreen@...omium.org>
Cc:     linux-kernel@...r.kernel.org, corbet@....net,
        linux-integrity@...r.kernel.org, gwendal@...omium.org,
        dianders@...omium.org, apronin@...omium.org,
        Pavel Machek <pavel@....cz>, Ben Boeckel <me@...boeckel.net>,
        rjw@...ysocki.net, jejb@...ux.ibm.com,
        Kees Cook <keescook@...omium.org>, dlunev@...gle.com,
        zohar@...ux.ibm.com, Matthew Garrett <mgarrett@...ora.tech>,
        jarkko@...nel.org, linux-pm@...r.kernel.org,
        Matthew Garrett <mjg59@...gle.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Peter Huewe <peterhuewe@....de>, axelj <axelj@...s.com>
Subject: Re: [PATCH v5 01/11] tpm: Add support for in-kernel resetting of PCRs

On Fri, Nov 11, 2022 at 03:16:26PM -0800, Evan Green wrote:
> diff --git a/drivers/char/tpm/tpm-interface.c b/drivers/char/tpm/tpm-interface.c
> index 1621ce8187052c..886277b2654e3b 100644
> --- a/drivers/char/tpm/tpm-interface.c
> +++ b/drivers/char/tpm/tpm-interface.c
> @@ -342,6 +342,53 @@ int tpm_pcr_extend(struct tpm_chip *chip, u32 pcr_idx,
>  }
>  EXPORT_SYMBOL_GPL(tpm_pcr_extend);
>  
> +/**
> + * tpm2_pcr_reset - Reset the specified PCR

Should this function be in drivers/char/tpm/tpm2-cmd.c instead of here?

> + * @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 tpm2_pcr_reset(struct tpm_chip *chip, u32 pcr_idx)

The callers of this function assume it returns a negative errno value.  But
actually it can return positive TPM2_RC_* error codes as well.  Probably you
should make it only return negative errno values.

> +{
> +	struct tpm2_null_auth_area auth_area;
> +	struct tpm_buf buf;
> +	int rc;
> +
> +	chip = tpm_find_get_ops(chip);
> +	if (!chip)
> +		return -ENODEV;
> +
> +	if (!(chip->flags & TPM_CHIP_FLAG_TPM2)) {
> +		rc = -ENOTTY;
> +		goto out;
> +	}
> +
> +	rc = tpm_buf_init(&buf, TPM2_ST_SESSIONS, TPM2_CC_PCR_RESET);
> +	if (rc)
> +		goto out;
> +
> +	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));

sizeof(struct tpm2_null_auth_area) => sizeof(auth_area)

> +	tpm_buf_append(&buf, (const unsigned char *)&auth_area,

unsigned char => u8

Also, since the code to append a "null" authorization area appears in both
tpm2_pcr_reset() and tpm2_pcr_extend(), perhaps it should be refactored into a
helper function?

> diff --git a/include/linux/tpm.h b/include/linux/tpm.h
> index dfeb25a0362dee..70134e6551745f 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,
> @@ -293,6 +294,13 @@ struct tpm_header {
>  	};
>  } __packed;
>  
> +struct tpm2_null_auth_area {
> +	__be32  handle;
> +	__be16  nonce_size;
> +	u8  attributes;
> +	__be16  auth_size;
> +} __packed;

struct tpm2_null_auth_area is only used by code in drivers/char/tpm/, so should
its declaration go in the internal header drivers/char/tpm/tpm.h instead?

- Eric