Re: [PATCH v4] tpm: Mask TPM RC in tpm2_start_auth_session()

[Stefano Garzarella]

On Mon, Apr 07, 2025 at 03:28:05PM +0300, Jarkko Sakkinen wrote:
> tpm2_start_auth_session() does not mask TPM RC correctly from the callers:
>
> [   28.766528] tpm tpm0: A TPM error (2307) occurred start auth session
>
> Process TPM RCs inside tpm2_start_auth_session(), and map them to POSIX
> error codes.
>
> Cc: stable@xxxxxxxxxxxxxxx # v6.10+
> Fixes: 699e3efd6c64 ("tpm: Add HMAC session start and end functions")
> Reported-by: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
> Closes: https://lore.kernel.org/linux-integrity/Z_NgdRHuTKP6JK--@xxxxxxxxxxxxxxxxxxx/
> Signed-off-by: Jarkko Sakkinen <jarkko@xxxxxxxxxx>
> ---
> v4:
> - tpm_to_ret()
> v3:
> - rc > 0
> v2:
> - Investigate TPM rc only after destroying tpm_buf.
> ---
> drivers/char/tpm/tpm2-sessions.c | 20 ++++++--------------
> include/linux/tpm.h              | 21 +++++++++++++++++++++
> 2 files changed, 27 insertions(+), 14 deletions(-)
>
> diff --git a/drivers/char/tpm/tpm2-sessions.c b/drivers/char/tpm/tpm2-sessions.c
> index 3f89635ba5e8..102e099f22c1 100644
> --- a/drivers/char/tpm/tpm2-sessions.c
> +++ b/drivers/char/tpm/tpm2-sessions.c
> @@ -40,11 +40,6 @@
>  *
>  * These are the usage functions:
>  *
> - * tpm2_start_auth_session() which allocates the opaque auth structure
> - *	and gets a session from the TPM.  This must be called before
> - *	any of the following functions.  The session is protected by a
> - *	session_key which is derived from a random salt value
> - *	encrypted to the NULL seed.
>  * tpm2_end_auth_session() kills the session and frees the resources.
>  *	Under normal operation this function is done by
>  *	tpm_buf_check_hmac_response(), so this is only to be used on
> @@ -963,16 +958,13 @@ static int tpm2_load_null(struct tpm_chip *chip, u32 *null_key)
> }
>
> /**
> - * tpm2_start_auth_session() - create a HMAC authentication session with the TPM
> - * @chip: the TPM chip structure to create the session with
> + * tpm2_start_auth_session() - Create an a HMAC authentication session
> + * @chip:	A TPM chip
>  *
> - * This function loads the NULL seed from its saved context and starts
> - * an authentication session on the null seed, fills in the
> - * @chip->auth structure to contain all the session details necessary
> - * for performing the HMAC, encrypt and decrypt operations and
> - * returns.  The NULL seed is flushed before this function returns.
> + * Loads the ephemeral key (null seed), and starts an HMAC authenticated
> + * session. The null seed is flushed before the return.
>  *
> - * Return: zero on success or actual error encountered.
> + * Returns zero on success, or a POSIX error code.
>  */
> int tpm2_start_auth_session(struct tpm_chip *chip)
> {
> @@ -1024,7 +1016,7 @@ int tpm2_start_auth_session(struct tpm_chip *chip)
> 	/* hash algorithm for session */
> 	tpm_buf_append_u16(&buf, TPM_ALG_SHA256);
>
> -	rc = tpm_transmit_cmd(chip, &buf, 0, "start auth session");
> +	rc = tpm_to_ret(tpm_transmit_cmd(chip, &buf, 0, "StartAuthSession"));
> 	tpm2_flush_context(chip, null_key);
>
> 	if (rc == TPM2_RC_SUCCESS)
> diff --git a/include/linux/tpm.h b/include/linux/tpm.h
> index 6c3125300c00..c826d5a9d894 100644
> --- a/include/linux/tpm.h
> +++ b/include/linux/tpm.h
> @@ -257,8 +257,29 @@ enum tpm2_return_codes {
> 	TPM2_RC_TESTING		= 0x090A, /* RC_WARN */
> 	TPM2_RC_REFERENCE_H0	= 0x0910,
> 	TPM2_RC_RETRY		= 0x0922,
> +	TPM2_RC_SESSION_MEMORY	= 0x0903,

nit: the other values are in ascending order, should we keep it or is it 
not important?

(more a question for me than for the patch)

> };
>
> +/*
> + * Convert a return value from tpm_transmit_cmd() to a POSIX return value. The
> + * fallback return value is -EFAULT.
> + */
> +static inline ssize_t tpm_to_ret(ssize_t ret)
> +{
> +	/* Already a POSIX error: */
> +	if (ret < 0)
> +		return ret;
> +
> +	switch (ret) {
> +	case TPM2_RC_SUCCESS:
> +		return 0;
> +	case TPM2_RC_SESSION_MEMORY:
> +		return -ENOMEM;
> +	default:
> +		return -EFAULT;
> +	}
> +}

I like this and in the future we could reuse it in different places like 
tpm2_load_context() and tpm2_save_context().

Reviewed-by: Stefano Garzarella <sgarzare@xxxxxxxxxx>


BTW for my understading, looking at that code (sorry if the answer is 
obvious, but I'm learning) I'm confused about the use of 
tpm2_rc_value().

For example in tpm2_load_context() we have:

    	rc = tpm_transmit_cmd(chip, &tbuf, 4, NULL);
    	...
	} else if (tpm2_rc_value(rc) == TPM2_RC_HANDLE ||
		   rc == TPM2_RC_REFERENCE_H0) {

While in tpm2_save_context(), we have:

	rc = tpm_transmit_cmd(chip, &tbuf, 0, NULL);
	...
	} else if (tpm2_rc_value(rc) == TPM2_RC_REFERENCE_H0) {

So to check TPM2_RC_REFERENCE_H0 we are using tpm2_rc_value() only 
sometimes, what's the reason?

Thanks,
Stefano