Re: [PATCH v4 06/13] tpm: add buffer function to point to returned parameters

From: Stefan Berger <stefanb@linux.ibm.com>
To: James Bottomley <James.Bottomley@HansenPartnership.com>,
	linux-integrity@vger.kernel.org
Cc: Jarkko Sakkinen <jarkko@kernel.org>,
	keyrings@vger.kernel.org, Ard Biesheuvel <ardb@kernel.org>
Subject: Re: [PATCH v4 06/13] tpm: add buffer function to point to returned parameters
Date: Tue, 2 May 2023 10:09:57 -0400	[thread overview]
Message-ID: <d5c86f6f-6854-c649-d2a9-8090cc7d74cd@linux.ibm.com> (raw)
In-Reply-To: <20230403214003.32093-7-James.Bottomley@HansenPartnership.com>

On 4/3/23 17:39, James Bottomley wrote:
> Introducing encryption sessions changes where the return parameters
> are located in the buffer because if a return session is present
> they're 4 bytes beyond the header with those 4 bytes showing the
> parameter length.  If there is no return session, then they're in the
> usual place immediately after the header.  The tpm_buf_parameters()
> encapsulates this calculation and should be used everywhere
> &buf.data[TPM_HEADER_SIZE] is used now.
> 
> Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>
> 
> ---
> v4: add kdoc
> ---
>   drivers/char/tpm/tpm-buf.c | 27 +++++++++++++++++++++++++++
>   include/linux/tpm.h        |  2 ++
>   2 files changed, 29 insertions(+)
> 
> diff --git a/drivers/char/tpm/tpm-buf.c b/drivers/char/tpm/tpm-buf.c
> index da0f6e725c3f..d107321bcdff 100644
> --- a/drivers/char/tpm/tpm-buf.c
> +++ b/drivers/char/tpm/tpm-buf.c
> @@ -277,3 +277,30 @@ u32 tpm_get_inc_u32(const u8 **ptr)
>   	return val;
>   }
>   EXPORT_SYMBOL_GPL(tpm_get_inc_u32);
> +
> +static u16 tpm_buf_tag(struct tpm_buf *buf)
> +{
> +	struct tpm_header *head = (struct tpm_header *)buf->data;
> +
> +	return be16_to_cpu(head->tag);
> +}
> +
> +/**
> + * tpm_buf_parameters - return the parameters area of the tpm_buf

in a TPM response

> + * @buf: tpm_buf to use
> + *
> + * Where the parameters are located depends on the tag of a TPM
> + * command. Evaluate this and return a pointer to the first byte of
> + * the parameters area.

I would also describe here what the non-obvious 4 bytes are that you are skipping over in case of TPM2_ST_SESSIONS.

With this:

Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>

> + *
> + * @return: pointer to parameters area
> + */
> +u8 *tpm_buf_parameters(struct tpm_buf *buf)
> +{
> +	int offset = TPM_HEADER_SIZE;
> +
> +	if (tpm_buf_tag(buf) == TPM2_ST_SESSIONS)
> +		offset += 4;
> +
> +	return &buf->data[offset];
> +}
> diff --git a/include/linux/tpm.h b/include/linux/tpm.h
> index 845eadfed715..d2fea2afd37c 100644
> --- a/include/linux/tpm.h
> +++ b/include/linux/tpm.h
> @@ -340,6 +340,8 @@ u8 tpm_get_inc_u8(const u8 **ptr);
>   u16 tpm_get_inc_u16(const u8 **ptr);
>   u32 tpm_get_inc_u32(const u8 **ptr);
>   
> +u8 *tpm_buf_parameters(struct tpm_buf *buf);
> +
>   /*
>    * Check if TPM device is in the firmware upgrade mode.
>    */