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

[Stefano Garzarella]

On Mon, Apr 07, 2025 at 02:30:05PM +0300, Jarkko Sakkinen wrote:
> 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@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>
> > > ---
> > > 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"?

Ooops, I thought `rc` was unsigned always returning TPM2_RC_* values,
but it looks it's not the case.

I meant the switch "block" inside the if block, at the end exactly what
you did in tpm_to_ret() in v4 :-)

Thanks,
Stefano