linux-kernel - Re: [PATCH v3] tpm: Mask TPM RC in tpm2_start_auth

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Message-ID: <rxygujdrx3d22mv2jcbp2mjr52noyf6c346vi7iljjf4ova2wz@zkoqxvcudi35>
Date: Mon, 7 Apr 2025 15:20:05 +0200
From: Stefano Garzarella <sgarzare@...hat.com>
To: Jarkko Sakkinen <jarkko@...nel.org>
Cc: keyrings@...r.kernel.org, stable@...r.kernel.org, 
	Herbert Xu <herbert@...dor.apana.org.au>, David Howells <dhowells@...hat.com>, 
	Lukas Wunner <lukas@...ner.de>, Ignat Korchagin <ignat@...udflare.com>, 
	"David S. Miller" <davem@...emloft.net>, Peter Huewe <peterhuewe@....de>, Jason Gunthorpe <jgg@...pe.ca>, 
	Ard Biesheuvel <ardb@...nel.org>, James Bottomley <James.Bottomley@...senpartnership.com>, 
	linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org, linux-integrity@...r.kernel.org
Subject: Re: [PATCH v3] tpm: Mask TPM RC in tpm2_start_auth_session()

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@...r.kernel.org # v6.10+
>> > Fixes: 699e3efd6c64 ("tpm: Add HMAC session start and end functions")
>> > Reported-by: Herbert Xu <herbert@...dor.apana.org.au>
>> > Closes: https://lore.kernel.org/linux-integrity/Z_NgdRHuTKP6JK--@gondor.apana.org.au/
>> > Signed-off-by: Jarkko Sakkinen <jarkko@...nel.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"?

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