tpm: Add tpm_buf_read_{u8,u16,u32}

Declare reader functions for the instances of struct tpm_buf. If the read
goes out of boundary, TPM_BUF_BOUNDARY_ERROR is set, and subsequent read
will do nothing.
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>
Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Tested-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>

drivers/char/tpm/tpm-buf.c

@@ -108,7 +108,7 @@ void tpm_buf_append(struct tpm_buf *buf, const u8 *new_data, u16 new_length)
 		return;
 
 	if ((buf->length + new_length) > PAGE_SIZE) {
-		WARN(1, "tpm_buf: overflow\n");
+		WARN(1, "tpm_buf: write overflow\n");
 		buf->flags |= TPM_BUF_OVERFLOW;
 		return;
 	}
@@ -144,3 +144,80 @@ 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);
+
+/**
+ * tpm_buf_read() - Read from a TPM buffer
+ * @buf:	&tpm_buf instance
+ * @offset:	offset within the buffer
+ * @count:	the number of bytes to read
+ * @output:	the output buffer
+ */
+static void tpm_buf_read(struct tpm_buf *buf, off_t *offset, size_t count, void *output)
+{
+	off_t next_offset;
+
+	/* Return silently if overflow has already happened. */
+	if (buf->flags & TPM_BUF_BOUNDARY_ERROR)
+		return;
+
+	next_offset = *offset + count;
+	if (next_offset > buf->length) {
+		WARN(1, "tpm_buf: read out of boundary\n");
+		buf->flags |= TPM_BUF_BOUNDARY_ERROR;
+		return;
+	}
+
+	memcpy(output, &buf->data[*offset], count);
+	*offset = next_offset;
+}
+
+/**
+ * tpm_buf_read_u8() - Read 8-bit word from a TPM buffer
+ * @buf:	&tpm_buf instance
+ * @offset:	offset within the buffer
+ *
+ * Return: next 8-bit word
+ */
+u8 tpm_buf_read_u8(struct tpm_buf *buf, off_t *offset)
+{
+	u8 value;
+
+	tpm_buf_read(buf, offset, sizeof(value), &value);
+
+	return value;
+}
+EXPORT_SYMBOL_GPL(tpm_buf_read_u8);
+
+/**
+ * tpm_buf_read_u16() - Read 16-bit word from a TPM buffer
+ * @buf:	&tpm_buf instance
+ * @offset:	offset within the buffer
+ *
+ * Return: next 16-bit word
+ */
+u16 tpm_buf_read_u16(struct tpm_buf *buf, off_t *offset)
+{
+	u16 value;
+
+	tpm_buf_read(buf, offset, sizeof(value), &value);
+
+	return be16_to_cpu(value);
+}
+EXPORT_SYMBOL_GPL(tpm_buf_read_u16);
+
+/**
+ * tpm_buf_read_u32() - Read 32-bit word from a TPM buffer
+ * @buf:	&tpm_buf instance
+ * @offset:	offset within the buffer
+ *
+ * Return: next 32-bit word
+ */
+u32 tpm_buf_read_u32(struct tpm_buf *buf, off_t *offset)
+{
+	u32 value;
+
+	tpm_buf_read(buf, offset, sizeof(value), &value);
+
+	return be32_to_cpu(value);
+}
+EXPORT_SYMBOL_GPL(tpm_buf_read_u32);

include/linux/tpm.h

@@ -302,6 +302,8 @@ enum tpm_buf_flags {
 	TPM_BUF_OVERFLOW	= BIT(0),
 	/* TPM2B format: */
 	TPM_BUF_TPM2B		= BIT(1),
+	/* read out of boundary: */
+	TPM_BUF_BOUNDARY_ERROR	= BIT(2),
 };
 
 /*
@@ -338,6 +340,9 @@ void tpm_buf_append(struct tpm_buf *buf, const u8 *new_data, u16 new_length);
 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);
+u8 tpm_buf_read_u8(struct tpm_buf *buf, off_t *offset);
+u16 tpm_buf_read_u16(struct tpm_buf *buf, off_t *offset);
+u32 tpm_buf_read_u32(struct tpm_buf *buf, off_t *offset);
 
 /*
  * Check if TPM device is in the firmware upgrade mode.