linux-kernel - Re: [PATCH v3 5/6] tpm: Add tpm_buf_read

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Date:   Mon, 23 Oct 2023 20:38:30 -0500
From:   Mario Limonciello <mario.limonciello@....com>
To:     Jarkko Sakkinen <jarkko@...nel.org>,
        linux-integrity@...r.kernel.org
Cc:     keyrings@...r.kernel.org,
        James Bottomley <James.Bottomley@...senPartnership.com>,
        William Roberts <bill.c.roberts@...il.com>,
        Stefan Berger <stefanb@...ux.ibm.com>,
        David Howells <dhowells@...hat.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Mimi Zohar <zohar@...ux.ibm.com>,
        Peter Huewe <peterhuewe@....de>,
        Julien Gomes <julien@...sta.com>,
        Jerry Snitselaar <jsnitsel@...hat.com>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 5/6] tpm: Add tpm_buf_read_{u8,u16,u32}

On 10/23/2023 20:15, Jarkko Sakkinen wrote:
> Add tpm_buf_read_u8(), tpm_buf_read_u16() and tpm_read_u32() for the sake
> of more convenient parsing of TPM responses.
> 
> Signed-off-by: Jarkko Sakkinen <jarkko@...nel.org>
> ---
>   drivers/char/tpm/tpm-buf.c | 69 ++++++++++++++++++++++++++++++++++++++
>   include/linux/tpm.h        |  3 ++
>   2 files changed, 72 insertions(+)
> 
> diff --git a/drivers/char/tpm/tpm-buf.c b/drivers/char/tpm/tpm-buf.c
> index f1d92d7e758d..bcd3cbcd9dd9 100644
> --- a/drivers/char/tpm/tpm-buf.c
> +++ b/drivers/char/tpm/tpm-buf.c
> @@ -124,3 +124,72 @@ void tpm_buf_append_u32(struct tpm_buf *buf, const u32 value)
>   	tpm_buf_append(buf, (u8 *)&value2, 4);
>   }
>   EXPORT_SYMBOL_GPL(tpm_buf_append_u32);
> +
> +/**
> + * tpm_buf_read() - Read from a TPM buffer
> + * @buf:	&tpm_buf instance
> + * @offset:	offset within the buffer
> + * @count:	the number of bytes to read
> + * @output:	the output buffer
> + */
> +static void tpm_buf_read(const struct tpm_buf *buf, off_t *offset, size_t count, void *output)
> +{
> +	if (*(offset + count) >= buf->length) {
> +		WARN(1, "tpm_buf: overflow\n");
> +		return;
> +	}

In the overflow case wouldn't you want to pass an error code up instead 
of just showing a WARN trace?

The helper functions can't tell the difference, and the net outcome is 
going to be that if there is overflow you get a warning trace in the 
kernel log and whatever garbage "value" happened to have going to the 
caller.  It's a programmer error but it's also unpredictable what 
happens here.

I think it's cleaner to have callers of 
tpm_buf_read_u8/tpm_buf_read_u16/tpm_buf_read_u32 to to be able to know 
something wrong happened.

> +
> +	memcpy(output, &buf->data[*offset], count);
> +	*offset += count;
> +}
> +
> +/**
> + * tpm_buf_read_u8() - Read 8-bit word from a TPM buffer
> + * @buf:	&tpm_buf instance
> + * @offset:	offset within the buffer
> + *
> + * Return: next 8-bit word
> + */
> +u8 tpm_buf_read_u8(const struct tpm_buf *buf, off_t *offset)
> +{
> +	u8 value;
> +
> +	tpm_buf_read(buf, offset, sizeof(value), &value);
> +
> +	return value;
> +}
> +EXPORT_SYMBOL_GPL(tpm_buf_read_u8);
> +
> +/**
> + * tpm_buf_read_u16() - Read 16-bit word from a TPM buffer
> + * @buf:	&tpm_buf instance
> + * @offset:	offset within the buffer
> + *
> + * Return: next 16-bit word
> + */
> +u16 tpm_buf_read_u16(const struct tpm_buf *buf, off_t *offset)
> +{
> +	u16 value;
> +
> +	tpm_buf_read(buf, offset, sizeof(value), &value);
> +
> +	return be16_to_cpu(value);
> +}
> +EXPORT_SYMBOL_GPL(tpm_buf_read_u16);
> +
> +/**
> + * tpm_buf_read_u32() - Read 32-bit word from a TPM buffer
> + * @buf:	&tpm_buf instance
> + * @offset:	offset within the buffer
> + *
> + * Return: next 32-bit word
> + */
> +u32 tpm_buf_read_u32(const struct tpm_buf *buf, off_t *offset)
> +{
> +	u32 value;
> +
> +	tpm_buf_read(buf, offset, sizeof(value), &value);
> +
> +	return be32_to_cpu(value);
> +}
> +EXPORT_SYMBOL_GPL(tpm_buf_read_u32);
> diff --git a/include/linux/tpm.h b/include/linux/tpm.h
> index 687b5173bdab..6590bd1f0a0e 100644
> --- a/include/linux/tpm.h
> +++ b/include/linux/tpm.h
> @@ -337,6 +337,9 @@ void tpm_buf_append(struct tpm_buf *buf, const u8 *new_data, u16 new_length);
>   void tpm_buf_append_u8(struct tpm_buf *buf, const u8 value);
>   void tpm_buf_append_u16(struct tpm_buf *buf, const u16 value);
>   void tpm_buf_append_u32(struct tpm_buf *buf, const u32 value);
> +u8 tpm_buf_read_u8(const struct tpm_buf *buf, off_t *offset);
> +u16 tpm_buf_read_u16(const struct tpm_buf *buf, off_t *offset);
> +u32 tpm_buf_read_u32(const struct tpm_buf *buf, off_t *offset);
>   
>   /*
>    * Check if TPM device is in the firmware upgrade mode.