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SubjectRe: [PATCH v3 5/6] tpm: Add tpm_buf_read_{u8,u16,u32}
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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@kernel.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.

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