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

Submitted by Roberto Sassu on June 21, 2017, 2:29 p.m.

Details

Message ID 20170621142941.32674-2-roberto.sassu@huawei.com
State New
Headers show

Commit Message

Roberto Sassu June 21, 2017, 2:29 p.m.
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>
---
 drivers/char/tpm/tpm2-cmd.c | 54 +++++++++++++++++++++++++++------------------
 1 file changed, 32 insertions(+), 22 deletions(-)

Comments

Jarkko Sakkinen June 22, 2017, 10:14 a.m.
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

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Roberto Sassu June 22, 2017, 11:54 a.m.
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@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.

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
>
Jarkko Sakkinen June 23, 2017, 10:56 a.m.
On Thu, Jun 22, 2017 at 01:54:29PM +0200, Roberto Sassu wrote:
> 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@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.
> 
> This should be done in the next patch, because tpm2_pcrread_header
> and tpm2_pcr_read_in are still used by tpm2_do_selftest().

OK, understood.

> > > +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.

Look at tpm_transmit_cmd(). It returns the TPM error. You return
positive number from this function it is a TPM error. Digest size
could be an additional parameter, yes.

This is how it works elsewhere in the subsystem so it would be also
coherent.

> > > +{
> > > +	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

You can declare it as u8 digest[] (or whatever the fiel name was).

Once the first two patches are in shape I'll test and apply them
to my tree so we step further with this.

/Jarkko

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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)
 };
 
+static int tpm2_pcr_read_tpm_buf(struct tpm_chip *chip, int pcr_idx,
+				 enum tpm2_algorithms algo, struct tpm_buf *buf,
+				 char *msg)
+{
+	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);
 	}
 
+	tpm_buf_destroy(&buf);
 	return rc;
 }