Re: [PATCH v3] tpm: Mask TPM RC in tpm2_start_auth_session()

[Jarkko Sakkinen <jarkko@kernel.org>]

On Mon, Apr 07, 2025 at 10:04:09AM +0200, Stefano Garzarella wrote:
> On Mon, Apr 07, 2025 at 10:20:57AM +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@vger.kernel.org # v6.10+
> > Fixes: 699e3efd6c64 ("tpm: Add HMAC session start and end functions")
> > Reported-by: Herbert Xu <herbert@gondor.apana.org.au>
> > Closes: https://lore.kernel.org/linux-integrity/Z_NgdRHuTKP6JK--@gondor.apana.org.au/
> > Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>
> > ---
> > v3:
> > - rc > 0
> > v2:
> > - Investigate TPM rc only after destroying tpm_buf.
> > ---
> > drivers/char/tpm/tpm2-sessions.c | 31 +++++++++++++++++--------------
> > include/linux/tpm.h              |  1 +
> > 2 files changed, 18 insertions(+), 14 deletions(-)
> > 
> > diff --git a/drivers/char/tpm/tpm2-sessions.c b/drivers/char/tpm/tpm2-sessions.c
> > index 3f89635ba5e8..abd54fb0a45a 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_transmit_cmd(chip, &buf, 0, "StartAuthSession");
> > 	tpm2_flush_context(chip, null_key);
> > 
> > 	if (rc == TPM2_RC_SUCCESS)
> > @@ -1032,6 +1024,17 @@ int tpm2_start_auth_session(struct tpm_chip *chip)
> > 
> > 	tpm_buf_destroy(&buf);
> > 
> > +	if (rc > 0) {
> 
> To avoid the nesting blocks, can we include `TPM2_RC_SUCCESS` case in the
> switch or move the `if (rc == TPM2_RC_SUCCESS)` before it?

What do you mean by "avoiding nesting blocks"?

> 
> Thanks,
> Stefano

BR, Jarkko