Re: [tpmdd-devel] [PATCH v3 1/6] tpm: use tpm_buf functions to perform a PCR read

Jarkko Sakkinen
2017-06-22T10:14:10Z

On Wed, Jun 21, 2017 at 04:29:36PM +0200, Roberto Sassu wrote:
> tpm2_pcr_read() now uses tpm_buf functions to build the TPM command
> to read a PCR. Those functions are preferred to passing a tpm2_cmd
> structure, as they provide protection against buffer overflow.
>
> Also, tpm2_pcr_read() code has been moved to tpm2_pcr_read_tpm_buf().
> Callers have to pass a tpm_buf structure, an algorithm supported by
> the TPM and call tpm_buf_destroy(). The algorithm still cannot be
> passed to the TPM driver interface. This parameter has been introduced
> for determining the digest size of a given algorithm.
>
> Moving tpm2_pcr_read() code to tpm2_pcr_read_tpm_buf() is necessary
> because callers of the new function obtain different information from
> the output buffer: tpm2_pcr_read() gets the digest, tpm2_do_selftest()
> will get the command return code and tpm2_get_pcr_allocation() will get
> the digest size.
>
> Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>

We want to migrate *everything* to use tpm_buf to the point that
tpm_transmit takes tpm_buf as parameter.

> ---
> drivers/char/tpm/tpm2-cmd.c | 54 +++++++++++++++++++++++++++------------------
> 1 file changed, 32 insertions(+), 22 deletions(-)
>
> diff --git a/drivers/char/tpm/tpm2-cmd.c b/drivers/char/tpm/tpm2-cmd.c
> index 3a99643..afd1b63 100644
> --- a/drivers/char/tpm/tpm2-cmd.c
> +++ b/drivers/char/tpm/tpm2-cmd.c
> @@ -231,15 +231,37 @@ static const u8 tpm2_ordinal_duration[TPM2_CC_LAST - TPM2_CC_FIRST + 1] = {
> (sizeof(struct tpm_input_header) + \
> sizeof(struct tpm2_pcr_read_in))
>
> -#define TPM2_PCR_READ_RESP_BODY_SIZE \
> - sizeof(struct tpm2_pcr_read_out)
> -
> static const struct tpm_input_header tpm2_pcrread_header = {
> .tag = cpu_to_be16(TPM2_ST_NO_SESSIONS),
> .length = cpu_to_be32(TPM2_PCR_READ_IN_SIZE),
> .ordinal = cpu_to_be32(TPM2_CC_PCR_READ)
> };

Remove this and move tpm2_pcr_read_out declaration before tpm2_pcr_read.
Its only a one shot helper structure for this function. You can take it
of from the union and delete tpm2_pcr_read_in completely.

> +static int tpm2_pcr_read_tpm_buf(struct tpm_chip *chip, int pcr_idx,
> + enum tpm2_algorithms algo, struct tpm_buf *buf,
> + char *msg)

This wrapper is unnecessary especially since the fallback path is still
in the responsiblity of the caller.

> +{
> + int rc;
> + u8 pcr_select[TPM2_PCR_SELECT_MIN] = {0};
> +
> + if (pcr_idx >= TPM2_PLATFORM_PCR)
> + return -EINVAL;
> +
> + rc = tpm_buf_init(buf, TPM2_ST_NO_SESSIONS, TPM2_CC_PCR_READ);
> + if (rc)
> + return rc;
> +
> + pcr_select[pcr_idx >> 3] = 1 << (pcr_idx & 0x7);
> +
> + tpm_buf_append_u32(buf, 1);
> + tpm_buf_append_u16(buf, algo);
> + tpm_buf_append_u8(buf, TPM2_PCR_SELECT_MIN);
> + tpm_buf_append(buf, (const unsigned char *)pcr_select,
> + sizeof(pcr_select));
> +
> + return tpm_transmit_cmd(chip, NULL, buf->data, PAGE_SIZE, 0, 0, msg);
> +}
> +
> /**
> * tpm2_pcr_read() - read a PCR value
> * @chip: TPM chip to use.
> @@ -251,29 +273,17 @@ static const struct tpm_input_header tpm2_pcrread_header = {
> int tpm2_pcr_read(struct tpm_chip *chip, int pcr_idx, u8 *res_buf)
> {
> int rc;
> - struct tpm2_cmd cmd;
> - u8 *buf;
> -
> - if (pcr_idx >= TPM2_PLATFORM_PCR)
> - return -EINVAL;
> -
> - cmd.header.in = tpm2_pcrread_header;
> - cmd.params.pcrread_in.pcr_selects_cnt = cpu_to_be32(1);
> - cmd.params.pcrread_in.hash_alg = cpu_to_be16(TPM2_ALG_SHA1);
> - cmd.params.pcrread_in.pcr_select_size = TPM2_PCR_SELECT_MIN;
> -
> - memset(cmd.params.pcrread_in.pcr_select, 0,
> - sizeof(cmd.params.pcrread_in.pcr_select));
> - cmd.params.pcrread_in.pcr_select[pcr_idx >> 3] = 1 << (pcr_idx & 0x7);
> + struct tpm_buf buf;
> + struct tpm2_pcr_read_out *out;
>
> - rc = tpm_transmit_cmd(chip, NULL, &cmd, sizeof(cmd),
> - TPM2_PCR_READ_RESP_BODY_SIZE,
> - 0, "attempting to read a pcr value");
> + rc = tpm2_pcr_read_tpm_buf(chip, pcr_idx, TPM2_ALG_SHA1, &buf,
> + "attempting to read a pcr value");
> if (rc == 0) {
> - buf = cmd.params.pcrread_out.digest;
> - memcpy(res_buf, buf, TPM_DIGEST_SIZE);
> + out = (struct tpm2_pcr_read_out *)&buf.data[TPM_HEADER_SIZE];
> + memcpy(res_buf, out->digest, TPM_DIGEST_SIZE);

I think that when changes are made that involve TPM_DIGEST_SIZE, it
should be simply replaced with SHA1_DIGEST_SIZE. It's just a constant
that makes reading the code harder.

> }
>
> + tpm_buf_destroy(&buf);
> return rc;
> }

With some adjustments this is a welcome change.

/Jarkko