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

[Roberto Sassu <roberto.sassu-hv44wF8Li93QT0dZR+AlfA@public.gmane.org>]

On 6/22/2017 12:14 PM, Jarkko Sakkinen wrote:
> 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-hv44wF8Li93QT0dZR+AlfA@public.gmane.org>
>
> 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.

This should be done in the next patch, because tpm2_pcrread_header
and tpm2_pcr_read_in are still used by tpm2_do_selftest().


>> +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.

Please have a look at patches 2/6 and 3/6. It will be more clear why
this change is necessary. Alternatively, instead of tpm_buf, I can add
the command return code and the digest size as parameters of this
function.


>> +{
>> +	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.

Ok. Should I also replace TPM_DIGEST_SIZE in tpm2_pcr_read_out
with SHA512_DIGEST_SIZE, given that the algorithm can be specified?

Thanks

Roberto


>>  	}
>>
>> +	tpm_buf_destroy(&buf);
>>  	return rc;
>>  }
>
> With some adjustments this is a welcome change.
>
> /Jarkko
>

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