Re: [PATCH v3 5/5] tpm: ensure that output of PCR read contains the correct digest size

[Mimi Zohar <zohar@linux.ibm.com>]

On Tue, 2018-10-30 at 16:47 +0100, Roberto Sassu wrote:
> This patch ensures that the digest size returned by the TPM during a PCR
> read matches the size of the algorithm passed as argument to
> tpm2_pcr_read(). The check is performed after information about the PCR
> banks has been retrieved.
> 
> Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
> ---
>  drivers/char/tpm/tpm2-cmd.c | 16 +++++++++++++++-
>  1 file changed, 15 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/char/tpm/tpm2-cmd.c b/drivers/char/tpm/tpm2-cmd.c
> index 8e821e7b4674..477dcc30fc53 100644
> --- a/drivers/char/tpm/tpm2-cmd.c
> +++ b/drivers/char/tpm/tpm2-cmd.c
> @@ -187,15 +187,28 @@ struct tpm2_pcr_read_out {
>  int tpm2_pcr_read(struct tpm_chip *chip, int pcr_idx,
>  		  struct tpm_digest *digest_struct, u16 *digest_size_ptr)
>  {
> +	int i;
>  	int rc;
>  	struct tpm_buf buf;
>  	struct tpm2_pcr_read_out *out;
>  	u8 pcr_select[TPM2_PCR_SELECT_MIN] = {0};
>  	u16 digest_size;
> +	u16 expected_digest_size = 0;
>  
>  	if (pcr_idx >= TPM2_PLATFORM_PCR)
>  		return -EINVAL;
>  
> +	if (!digest_size_ptr) {
> +		for (i = 0; i < ARRAY_SIZE(chip->active_banks) &&
> +		     chip->active_banks[i].alg_id != digest_struct->alg_id; i++)
> +			;
> +
> +		if (i == ARRAY_SIZE(chip->active_banks))
> +			return -EINVAL;
> +
> +		expected_digest_size = chip->active_banks[i].digest_size;
> +	}
> +
>  	rc = tpm_buf_init(&buf, TPM2_ST_NO_SESSIONS, TPM2_CC_PCR_READ);
>  	if (rc)
>  		return rc;
> @@ -215,7 +228,8 @@ int tpm2_pcr_read(struct tpm_chip *chip, int pcr_idx,
>  
>  	out = (struct tpm2_pcr_read_out *)&buf.data[TPM_HEADER_SIZE];
>  	digest_size = be16_to_cpu(out->digest_size);
> -	if (digest_size > sizeof(digest_struct->digest)) {
> +	if ((digest_size_ptr && digest_size > sizeof(digest_struct->digest)) ||

The returned digest size should never be larger than the structure
field.  The digest_size_ptr test is unnecessary.

Mimi

> +	    (!digest_size_ptr && digest_size != expected_digest_size)) {
>  		rc = -EINVAL;
>  		goto out;
>  	}