diff mbox series

[v4,04/13] tpm: add buffer handling for TPM2B types

Message ID 20230403214003.32093-5-James.Bottomley@HansenPartnership.com (mailing list archive)
State New
Headers show
Series add integrity and security to TPM2 transactions | expand

Commit Message

James Bottomley April 3, 2023, 9:39 p.m. UTC 
Most complex TPM commands require appending TPM2B buffers to the
command body.  TPM2B types are variable size arrays, making it
difficult to represent them as structures.  Introduce primitives to
build them up using in place buffer append operations.

Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>

---
v4: add kernel doc
---
 drivers/char/tpm/tpm-buf.c | 109 ++++++++++++++++++++++++++++++++-----
 include/linux/tpm.h        |   3 +
 2 files changed, 97 insertions(+), 15 deletions(-)

Comments

Jarkko Sakkinen April 23, 2023, 4:12 a.m. UTC | #1 
On Tue Apr 4, 2023 at 12:39 AM EEST, James Bottomley wrote:
> Most complex TPM commands require appending TPM2B buffers to the
> command body.  TPM2B types are variable size arrays, making it
> difficult to represent them as structures.  Introduce primitives to

"TPM2B is a buffer defined 16-bit size field followed by the blob."

Just state what it is, as "variable sized array" is ambiguous
terminology.

> build them up using in place buffer append operations.
>
> Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>
>
> ---
> v4: add kernel doc
> ---
>  drivers/char/tpm/tpm-buf.c | 109 ++++++++++++++++++++++++++++++++-----
>  include/linux/tpm.h        |   3 +
>  2 files changed, 97 insertions(+), 15 deletions(-)
>
> diff --git a/drivers/char/tpm/tpm-buf.c b/drivers/char/tpm/tpm-buf.c
> index 3351db515e6b..b7e42fb6266c 100644
> --- a/drivers/char/tpm/tpm-buf.c
> +++ b/drivers/char/tpm/tpm-buf.c
> @@ -6,27 +6,16 @@
>  #include <linux/module.h>
>  #include <linux/tpm.h>
>  
> -/**
> - * tpm_buf_init - initialize a TPM command buffer
> - * @buf: pointer to a tpm_buf structure (usually on stack)
> - * @tag: command tag
> - * @ordinal: command ordinal
> - *
> - * Allocates a 4k buffer to hold the command structure.
> - *
> - * @return: 0 on success or -ENOMEM
> - */
> -int tpm_buf_init(struct tpm_buf *buf, u16 tag, u32 ordinal)
> +static int __tpm_buf_init(struct tpm_buf *buf)
>  {
>  	buf->data = (u8 *)__get_free_page(GFP_KERNEL);
>  	if (!buf->data)
>  		return -ENOMEM;
>  
>  	buf->flags = 0;
> -	tpm_buf_reset(buf, tag, ordinal);
> +
>  	return 0;
>  }
> -EXPORT_SYMBOL_GPL(tpm_buf_init);
>  
>  /**
>   * tpm_buf_reset - reset an initialized TPM command buffer
> @@ -48,6 +37,58 @@ void tpm_buf_reset(struct tpm_buf *buf, u16 tag, u32 ordinal)
>  }
>  EXPORT_SYMBOL_GPL(tpm_buf_reset);
>  
> +/**
> + * tpm_buf_init - initialize a TPM command buffer
> + * @buf: pointer to a tpm_buf structure (usually on stack)
> + * @tag: command tag
> + * @ordinal: command ordinal
> + *
> + * Allocates a 4k buffer to hold the command structure.
> + *
> + * @return: 0 on success or -ENOMEM
> + */
> +int tpm_buf_init(struct tpm_buf *buf, u16 tag, u32 ordinal)
> +{
> +	int rc;
> +
> +	rc = __tpm_buf_init(buf);
> +	if (rc)
> +		return rc;
> +
> +	tpm_buf_reset(buf, tag, ordinal);
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL_GPL(tpm_buf_init);
> +
> +/**
> + * tpm_buf_init_2b - initialize a TPM command buffer for 2B data
> + *
> + * TPM commands are often composed of sets of TPM2B data.  This

Two undescriptive terms used for the same thing:

* 2B data
* TPM2B data

I'd pick only one, and something that is understable by common sense
instead of this gibberish.

I'd suggest along the lines of:

/*
 * tpm_buf_init_data - initialize a TPM data buffer
 *
 * TPM2 protocol uses data buffers (TPM2B_*) to carry blobs of different types.
 * Each buffer starts with 16-bit unsigned integer defining the size of the
 * blob that follows it.
 * ...

> + * function initializes a tpm_buf (@buf) as a holder of TPM2B data,
> + * which allows all the current tpm2_buf_appendX functions to work on it.

I don't understand what I'm reading, i.e. what literally happens
when something is initialized as a holder of something else...

Please make this understandable.

> + *
> + * @return: 0 on success or -ENOMEM
> + */
> +int tpm_buf_init_2b(struct tpm_buf *buf)
> +{
> +	struct tpm_header *head;
> +	int rc;
> +
> +	rc = __tpm_buf_init(buf);
> +	if (rc)
> +		return rc;
> +
> +	head = (struct tpm_header *) buf->data;
> +

I'd remove this empty line.
i
> +	head->length = cpu_to_be32(sizeof(*head));
> +

I'd remove this empty line.

I'm missing (not described here or in the commit message) how
can you use command header here? AFAIK, header for TPM2B_*
is 16-bit number.

Please document if something weird is done. I don't understand
this.

> +	buf->flags = TPM_BUF_2B;

empty line here

> +	return 0;
> +}
> +EXPORT_SYMBOL_GPL(tpm_buf_init_2b);
> +
>  /**
>   * tpm_buf_destroy - destroy an initialized TPM command buffer
>   * @buf: pointer to a tpm_buf structure (usually on stack)
> @@ -60,6 +101,13 @@ void tpm_buf_destroy(struct tpm_buf *buf)
>  }
>  EXPORT_SYMBOL_GPL(tpm_buf_destroy);
>  
> +static void *tpm_buf_data(struct tpm_buf *buf)
> +{
> +	if (buf->flags & TPM_BUF_2B)
> +		return buf->data + TPM_HEADER_SIZE;

empty line here

> +	return buf->data;
> +}
> +
>  /**
>   * tpm_buf_length - get the current length of a TPM command
>   * @buf: pointer to a tpm_buf structure (usually on stack)
> @@ -69,8 +117,12 @@ EXPORT_SYMBOL_GPL(tpm_buf_destroy);
>  u32 tpm_buf_length(struct tpm_buf *buf)
>  {
>  	struct tpm_header *head = (struct tpm_header *)buf->data;
> +	u32 len;
>  
> -	return be32_to_cpu(head->length);
> +	len = be32_to_cpu(head->length);
> +	if (buf->flags & TPM_BUF_2B)
> +		len -= sizeof(*head);

empty line here

> +	return len;
>  }
>  EXPORT_SYMBOL_GPL(tpm_buf_length);
>  
> @@ -88,7 +140,7 @@ void tpm_buf_append(struct tpm_buf *buf,
>  		    unsigned int new_len)
>  {
>  	struct tpm_header *head = (struct tpm_header *) buf->data;
> -	u32 len = tpm_buf_length(buf);
> +	u32 len = be32_to_cpu(head->length);
>  
>  	/* Return silently if overflow has already happened. */
>  	if (buf->flags & TPM_BUF_OVERFLOW)
> @@ -150,3 +202,30 @@ 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);
> +
> +static void tpm_buf_reset_int(struct tpm_buf *buf)
> +{
> +	struct tpm_header *head;
> +
> +	head = (struct tpm_header *)buf->data;
> +	head->length = cpu_to_be32(sizeof(*head));
> +}
> +
> +/**
> + * tpm_buf_append_2b - append TPM2B data to an initialized TPM command buffer
> + * @tpm2b: pointer to a tpm_buf structure containing the TPM2B data.
> + *
> + * Appends @tpm2b as a correct TPM2B structure (big endian short
> + * length) followed by data of that length.  @tpm2b is then emptied to
> + * allow reuse.
> + */
> +void tpm_buf_append_2b(struct tpm_buf *buf, struct tpm_buf *tpm2b)
> +{
> +	u16 len = tpm_buf_length(tpm2b);
> +
> +	tpm_buf_append_u16(buf, len);
> +	tpm_buf_append(buf, tpm_buf_data(tpm2b), len);
> +	/* clear the buf for reuse */
> +	tpm_buf_reset_int(tpm2b);
> +}
> +EXPORT_SYMBOL_GPL(tpm_buf_append_2b);
> diff --git a/include/linux/tpm.h b/include/linux/tpm.h
> index 9c9b5760b412..76d495cb5b08 100644
> --- a/include/linux/tpm.h
> +++ b/include/linux/tpm.h
> @@ -301,6 +301,7 @@ struct tpm_header {
>  
>  enum tpm_buf_flags {
>  	TPM_BUF_OVERFLOW	= BIT(0),
> +	TPM_BUF_2B		= BIT(1),
>  };
>  
>  struct tpm_buf {
> @@ -325,6 +326,7 @@ struct tpm2_hash {
>  
>  
>  int tpm_buf_init(struct tpm_buf *buf, u16 tag, u32 ordinal);
> +int tpm_buf_init_2b(struct tpm_buf *buf);
>  void tpm_buf_reset(struct tpm_buf *buf, u16 tag, u32 ordinal);
>  void tpm_buf_destroy(struct tpm_buf *buf);
>  u32 tpm_buf_length(struct tpm_buf *buf);
> @@ -333,6 +335,7 @@ void tpm_buf_append(struct tpm_buf *buf, const unsigned char *new_data,
>  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);
> +void tpm_buf_append_2b(struct tpm_buf *buf, struct tpm_buf *tpm2b);
>  
>  /*
>   * Check if TPM device is in the firmware upgrade mode.
> -- 
> 2.35.3

BR, Jarkko
Stefan Berger May 2, 2023, 3:43 p.m. UTC | #2 
On 4/3/23 17:39, James Bottomley wrote:
> Most complex TPM commands require appending TPM2B buffers to the
> command body.  TPM2B types are variable size arrays, making it
> difficult to represent them as structures.  Introduce primitives to
> build them up using in place buffer append operations.
> 
> Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>
> 
> ---
> v4: add kernel doc
> ---
>   drivers/char/tpm/tpm-buf.c | 109 ++++++++++++++++++++++++++++++++-----
>   include/linux/tpm.h        |   3 +
>   2 files changed, 97 insertions(+), 15 deletions(-)
> 
> diff --git a/drivers/char/tpm/tpm-buf.c b/drivers/char/tpm/tpm-buf.c
> index 3351db515e6b..b7e42fb6266c 100644
> --- a/drivers/char/tpm/tpm-buf.c
> +++ b/drivers/char/tpm/tpm-buf.c
> @@ -6,27 +6,16 @@
>   #include <linux/module.h>
>   #include <linux/tpm.h>
>   
> -/**
> - * tpm_buf_init - initialize a TPM command buffer
> - * @buf: pointer to a tpm_buf structure (usually on stack)
> - * @tag: command tag
> - * @ordinal: command ordinal
> - *
> - * Allocates a 4k buffer to hold the command structure.
> - *
> - * @return: 0 on success or -ENOMEM
> - */
> -int tpm_buf_init(struct tpm_buf *buf, u16 tag, u32 ordinal)
> +static int __tpm_buf_init(struct tpm_buf *buf)
>   {
>   	buf->data = (u8 *)__get_free_page(GFP_KERNEL);
>   	if (!buf->data)
>   		return -ENOMEM;
>   
>   	buf->flags = 0;
> -	tpm_buf_reset(buf, tag, ordinal);
> +
>   	return 0;
>   }
> -EXPORT_SYMBOL_GPL(tpm_buf_init);
>   
>   /**
>    * tpm_buf_reset - reset an initialized TPM command buffer
> @@ -48,6 +37,58 @@ void tpm_buf_reset(struct tpm_buf *buf, u16 tag, u32 ordinal)
>   }
>   EXPORT_SYMBOL_GPL(tpm_buf_reset);
>   
> +/**
> + * tpm_buf_init - initialize a TPM command buffer
> + * @buf: pointer to a tpm_buf structure (usually on stack)
> + * @tag: command tag
> + * @ordinal: command ordinal
> + *
> + * Allocates a 4k buffer to hold the command structure.
> + *
> + * @return: 0 on success or -ENOMEM
> + */
> +int tpm_buf_init(struct tpm_buf *buf, u16 tag, u32 ordinal)
> +{
> +	int rc;
> +
> +	rc = __tpm_buf_init(buf);
> +	if (rc)
> +		return rc;
> +
> +	tpm_buf_reset(buf, tag, ordinal);
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL_GPL(tpm_buf_init);
> +
> +/**
> + * tpm_buf_init_2b - initialize a TPM command buffer for 2B data
> + * @buf: pointer to a tpm_buf structure (usually on stack)
> + *
> + * TPM commands are often composed of sets of TPM2B data.  This
> + * function initializes a tpm_buf (@buf) as a holder of TPM2B data,
> + * which allows all the current tpm2_buf_appendX functions to work on it.
> + *
> + * @return: 0 on success or -ENOMEM
> + */
> +int tpm_buf_init_2b(struct tpm_buf *buf)
> +{
> +	struct tpm_header *head;
> +	int rc;
> +
> +	rc = __tpm_buf_init(buf);
> +	if (rc)
> +		return rc;
> +
> +	head = (struct tpm_header *) buf->data;
> +
> +	head->length = cpu_to_be32(sizeof(*head));


just tpm_buf_init(buf) and then set the flag below?


> +
> +	buf->flags = TPM_BUF_2B;
> +	return 0;
> +}
> +EXPORT_SYMBOL_GPL(tpm_buf_init_2b);
> +
>   /**
>    * tpm_buf_destroy - destroy an initialized TPM command buffer
>    * @buf: pointer to a tpm_buf structure (usually on stack)
> @@ -60,6 +101,13 @@ void tpm_buf_destroy(struct tpm_buf *buf)
>   }
>   EXPORT_SYMBOL_GPL(tpm_buf_destroy);
>   
> +static void *tpm_buf_data(struct tpm_buf *buf)
> +{
> +	if (buf->flags & TPM_BUF_2B)
> +		return buf->data + TPM_HEADER_SIZE;
> +	return buf->data;
> +}
> +
>   /**
>    * tpm_buf_length - get the current length of a TPM command
>    * @buf: pointer to a tpm_buf structure (usually on stack)
> @@ -69,8 +117,12 @@ EXPORT_SYMBOL_GPL(tpm_buf_destroy);
>   u32 tpm_buf_length(struct tpm_buf *buf)
>   {
>   	struct tpm_header *head = (struct tpm_header *)buf->data;
> +	u32 len;
>   
> -	return be32_to_cpu(head->length);
> +	len = be32_to_cpu(head->length);
> +	if (buf->flags & TPM_BUF_2B)
> +		len -= sizeof(*head);
> +	return len;
>   }
>   EXPORT_SYMBOL_GPL(tpm_buf_length);
>   
> @@ -88,7 +140,7 @@ void tpm_buf_append(struct tpm_buf *buf,
>   		    unsigned int new_len)
>   {
>   	struct tpm_header *head = (struct tpm_header *) buf->data;
> -	u32 len = tpm_buf_length(buf);
> +	u32 len = be32_to_cpu(head->length);
>   
>   	/* Return silently if overflow has already happened. */
>   	if (buf->flags & TPM_BUF_OVERFLOW)
> @@ -150,3 +202,30 @@ 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);
> +
> +static void tpm_buf_reset_int(struct tpm_buf *buf)
> +{
> +	struct tpm_header *head;
> +
> +	head = (struct tpm_header *)buf->data;
> +	head->length = cpu_to_be32(sizeof(*head));

tpm_buf_reset(buf, 0, 0);

in tpm_buf_reset() it seem there should be this:

buf->flags &= ~TPM_BUF_OVERFLOW;

> +}
> +
> +/**
> + * tpm_buf_append_2b - append TPM2B data to an initialized TPM command buffer
> + * @tpm2b: pointer to a tpm_buf structure containing the TPM2B data.
> + *
> + * Appends @tpm2b as a correct TPM2B structure (big endian short
> + * length) followed by data of that length.  @tpm2b is then emptied to
> + * allow reuse.
> + */
> +void tpm_buf_append_2b(struct tpm_buf *buf, struct tpm_buf *tpm2b)
> +{
> +	u16 len = tpm_buf_length(tpm2b);
> +

if (tpm2b->flags & TPM_BUF_OVERFLOW) {
     buf->flags |= TPM_BUF_OVERFLOW;
     return;
}

> +	tpm_buf_append_u16(buf, len);
> +	tpm_buf_append(buf, tpm_buf_data(tpm2b), len);


> +	/* clear the buf for reuse */
> +	tpm_buf_reset_int(tpm2b);
> +}
> +EXPORT_SYMBOL_GPL(tpm_buf_append_2b);
> diff --git a/include/linux/tpm.h b/include/linux/tpm.h
> index 9c9b5760b412..76d495cb5b08 100644
> --- a/include/linux/tpm.h
> +++ b/include/linux/tpm.h
> @@ -301,6 +301,7 @@ struct tpm_header {
>   
>   enum tpm_buf_flags {
>   	TPM_BUF_OVERFLOW	= BIT(0),
> +	TPM_BUF_2B		= BIT(1),
>   };
>   
>   struct tpm_buf {
> @@ -325,6 +326,7 @@ struct tpm2_hash {
>   
>   
>   int tpm_buf_init(struct tpm_buf *buf, u16 tag, u32 ordinal);
> +int tpm_buf_init_2b(struct tpm_buf *buf);
>   void tpm_buf_reset(struct tpm_buf *buf, u16 tag, u32 ordinal);
>   void tpm_buf_destroy(struct tpm_buf *buf);
>   u32 tpm_buf_length(struct tpm_buf *buf);
> @@ -333,6 +335,7 @@ void tpm_buf_append(struct tpm_buf *buf, const unsigned char *new_data,
>   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);
> +void tpm_buf_append_2b(struct tpm_buf *buf, struct tpm_buf *tpm2b);
>   
>   /*
>    * Check if TPM device is in the firmware upgrade mode.
Jarkko Sakkinen May 3, 2023, 11:29 a.m. UTC | #3 
On Tue May 2, 2023 at 6:43 PM EEST, Stefan Berger wrote:
> > +void tpm_buf_append_2b(struct tpm_buf *buf, struct tpm_buf *tpm2b)
> > +{
> > +	u16 len = tpm_buf_length(tpm2b);
> > +
>
> if (tpm2b->flags & TPM_BUF_OVERFLOW) {
>      buf->flags |= TPM_BUF_OVERFLOW;
>      return;
> }
>
> > +	tpm_buf_append_u16(buf, len);
> > +	tpm_buf_append(buf, tpm_buf_data(tpm2b), len);
>
>
> > +	/* clear the buf for reuse */
> > +	tpm_buf_reset_int(tpm2b);

This should only append. Unpredictable side-effect like this, will 
add by factors to the time spent maintaining all this.

For any extra steps, do them in the call site.

BR, Jarkko
diff mbox series

Patch

diff --git a/drivers/char/tpm/tpm-buf.c b/drivers/char/tpm/tpm-buf.c
index 3351db515e6b..b7e42fb6266c 100644
--- a/drivers/char/tpm/tpm-buf.c
+++ b/drivers/char/tpm/tpm-buf.c
@@ -6,27 +6,16 @@ 
 #include <linux/module.h>
 #include <linux/tpm.h>
 
-/**
- * tpm_buf_init - initialize a TPM command buffer
- * @buf: pointer to a tpm_buf structure (usually on stack)
- * @tag: command tag
- * @ordinal: command ordinal
- *
- * Allocates a 4k buffer to hold the command structure.
- *
- * @return: 0 on success or -ENOMEM
- */
-int tpm_buf_init(struct tpm_buf *buf, u16 tag, u32 ordinal)
+static int __tpm_buf_init(struct tpm_buf *buf)
 {
 	buf->data = (u8 *)__get_free_page(GFP_KERNEL);
 	if (!buf->data)
 		return -ENOMEM;
 
 	buf->flags = 0;
-	tpm_buf_reset(buf, tag, ordinal);
+
 	return 0;
 }
-EXPORT_SYMBOL_GPL(tpm_buf_init);
 
 /**
  * tpm_buf_reset - reset an initialized TPM command buffer
@@ -48,6 +37,58 @@  void tpm_buf_reset(struct tpm_buf *buf, u16 tag, u32 ordinal)
 }
 EXPORT_SYMBOL_GPL(tpm_buf_reset);
 
+/**
+ * tpm_buf_init - initialize a TPM command buffer
+ * @buf: pointer to a tpm_buf structure (usually on stack)
+ * @tag: command tag
+ * @ordinal: command ordinal
+ *
+ * Allocates a 4k buffer to hold the command structure.
+ *
+ * @return: 0 on success or -ENOMEM
+ */
+int tpm_buf_init(struct tpm_buf *buf, u16 tag, u32 ordinal)
+{
+	int rc;
+
+	rc = __tpm_buf_init(buf);
+	if (rc)
+		return rc;
+
+	tpm_buf_reset(buf, tag, ordinal);
+
+	return 0;
+}
+EXPORT_SYMBOL_GPL(tpm_buf_init);
+
+/**
+ * tpm_buf_init_2b - initialize a TPM command buffer for 2B data
+ * @buf: pointer to a tpm_buf structure (usually on stack)
+ *
+ * TPM commands are often composed of sets of TPM2B data.  This
+ * function initializes a tpm_buf (@buf) as a holder of TPM2B data,
+ * which allows all the current tpm2_buf_appendX functions to work on it.
+ *
+ * @return: 0 on success or -ENOMEM
+ */
+int tpm_buf_init_2b(struct tpm_buf *buf)
+{
+	struct tpm_header *head;
+	int rc;
+
+	rc = __tpm_buf_init(buf);
+	if (rc)
+		return rc;
+
+	head = (struct tpm_header *) buf->data;
+
+	head->length = cpu_to_be32(sizeof(*head));
+
+	buf->flags = TPM_BUF_2B;
+	return 0;
+}
+EXPORT_SYMBOL_GPL(tpm_buf_init_2b);
+
 /**
  * tpm_buf_destroy - destroy an initialized TPM command buffer
  * @buf: pointer to a tpm_buf structure (usually on stack)
@@ -60,6 +101,13 @@  void tpm_buf_destroy(struct tpm_buf *buf)
 }
 EXPORT_SYMBOL_GPL(tpm_buf_destroy);
 
+static void *tpm_buf_data(struct tpm_buf *buf)
+{
+	if (buf->flags & TPM_BUF_2B)
+		return buf->data + TPM_HEADER_SIZE;
+	return buf->data;
+}
+
 /**
  * tpm_buf_length - get the current length of a TPM command
  * @buf: pointer to a tpm_buf structure (usually on stack)
@@ -69,8 +117,12 @@  EXPORT_SYMBOL_GPL(tpm_buf_destroy);
 u32 tpm_buf_length(struct tpm_buf *buf)
 {
 	struct tpm_header *head = (struct tpm_header *)buf->data;
+	u32 len;
 
-	return be32_to_cpu(head->length);
+	len = be32_to_cpu(head->length);
+	if (buf->flags & TPM_BUF_2B)
+		len -= sizeof(*head);
+	return len;
 }
 EXPORT_SYMBOL_GPL(tpm_buf_length);
 
@@ -88,7 +140,7 @@  void tpm_buf_append(struct tpm_buf *buf,
 		    unsigned int new_len)
 {
 	struct tpm_header *head = (struct tpm_header *) buf->data;
-	u32 len = tpm_buf_length(buf);
+	u32 len = be32_to_cpu(head->length);
 
 	/* Return silently if overflow has already happened. */
 	if (buf->flags & TPM_BUF_OVERFLOW)
@@ -150,3 +202,30 @@  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);
+
+static void tpm_buf_reset_int(struct tpm_buf *buf)
+{
+	struct tpm_header *head;
+
+	head = (struct tpm_header *)buf->data;
+	head->length = cpu_to_be32(sizeof(*head));
+}
+
+/**
+ * tpm_buf_append_2b - append TPM2B data to an initialized TPM command buffer
+ * @tpm2b: pointer to a tpm_buf structure containing the TPM2B data.
+ *
+ * Appends @tpm2b as a correct TPM2B structure (big endian short
+ * length) followed by data of that length.  @tpm2b is then emptied to
+ * allow reuse.
+ */
+void tpm_buf_append_2b(struct tpm_buf *buf, struct tpm_buf *tpm2b)
+{
+	u16 len = tpm_buf_length(tpm2b);
+
+	tpm_buf_append_u16(buf, len);
+	tpm_buf_append(buf, tpm_buf_data(tpm2b), len);
+	/* clear the buf for reuse */
+	tpm_buf_reset_int(tpm2b);
+}
+EXPORT_SYMBOL_GPL(tpm_buf_append_2b);
diff --git a/include/linux/tpm.h b/include/linux/tpm.h
index 9c9b5760b412..76d495cb5b08 100644
--- a/include/linux/tpm.h
+++ b/include/linux/tpm.h
@@ -301,6 +301,7 @@  struct tpm_header {
 
 enum tpm_buf_flags {
 	TPM_BUF_OVERFLOW	= BIT(0),
+	TPM_BUF_2B		= BIT(1),
 };
 
 struct tpm_buf {
@@ -325,6 +326,7 @@  struct tpm2_hash {
 
 
 int tpm_buf_init(struct tpm_buf *buf, u16 tag, u32 ordinal);
+int tpm_buf_init_2b(struct tpm_buf *buf);
 void tpm_buf_reset(struct tpm_buf *buf, u16 tag, u32 ordinal);
 void tpm_buf_destroy(struct tpm_buf *buf);
 u32 tpm_buf_length(struct tpm_buf *buf);
@@ -333,6 +335,7 @@  void tpm_buf_append(struct tpm_buf *buf, const unsigned char *new_data,
 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);
+void tpm_buf_append_2b(struct tpm_buf *buf, struct tpm_buf *tpm2b);
 
 /*
  * Check if TPM device is in the firmware upgrade mode.