tpm: introduce tpm_buf

This patch introduces struct tpm_buf that provides a string buffer for
constructing TPM commands. This allows to construct variable sized TPM
commands. For the buffer a page is allocated and mapped, which limits
maximum size to PAGE_SIZE.

Variable sized TPM commands are needed in order to add algorithmic
agility.
Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Reviewed-by: Peter Huewe <peterhuewe@gmx.de>
Signed-off-by: Peter Huewe <peterhuewe@gmx.de>

drivers/char/tpm/tpm.h

 /*
  * Copyright (C) 2004 IBM Corporation
+ * Copyright (C) 2015 Intel Corporation
  *
  * Authors:
  * Leendert van Doorn <leendert@watson.ibm.com>
@@ -28,6 +29,7 @@
 #include <linux/tpm.h>
 #include <linux/acpi.h>
 #include <linux/cdev.h>
+#include <linux/highmem.h>
 
 enum tpm_const {
 	TPM_MINOR = 224,	/* officially assigned */
@@ -390,6 +392,101 @@ struct tpm_cmd_t {
 	tpm_cmd_params	params;
 } __packed;
 
+/* A string buffer type for constructing TPM commands. This is based on the
+ * ideas of string buffer code in security/keys/trusted.h but is heap based
+ * in order to keep the stack usage minimal.
+ */
+
+enum tpm_buf_flags {
+	TPM_BUF_OVERFLOW	= BIT(0),
+};
+
+struct tpm_buf {
+	struct page *data_page;
+	unsigned int flags;
+	u8 *data;
+};
+
+static inline void tpm_buf_init(struct tpm_buf *buf, u16 tag, u32 ordinal)
+{
+	struct tpm_input_header *head;
+
+	buf->data_page = alloc_page(GFP_HIGHUSER);
+	if (!buf->data_page)
+		return -ENOMEM;
+
+	buf->flags = 0;
+	buf->data = kmap(buf->data_page);
+
+	head = (struct tpm_input_header *) buf->data;
+
+	head->tag = cpu_to_be16(tag);
+	head->length = cpu_to_be32(sizeof(*head));
+	head->ordinal = cpu_to_be32(ordinal);
+
+	return 0;
+}
+
+static inline void tpm_buf_destroy(struct tpm_buf *buf)
+{
+	kunmap(buf->data_page);
+	__free_page(buf->data_page);
+}
+
+static inline u32 tpm_buf_length(struct tpm_buf *buf)
+{
+	struct tpm_input_header *head = (struct tpm_input_header *) buf->data;
+
+	return be32_to_cpu(head->length);
+}
+
+static inline u16 tpm_buf_tag(struct tpm_buf *buf)
+{
+	struct tpm_input_header *head = (struct tpm_input_header *) buf->data;
+
+	return be16_to_cpu(head->tag);
+}
+
+static inline void tpm_buf_append(struct tpm_buf *buf,
+				  const unsigned char *new_data,
+				  unsigned int new_len)
+{
+	struct tpm_input_header *head = (struct tpm_input_header *) buf->data;
+	u32 len = tpm_buf_length(buf);
+
+	/* Return silently if overflow has already happened. */
+	if (buf->flags & TPM_BUF_OVERFLOW)
+		return;
+
+	if ((len + new_len) > PAGE_SIZE) {
+		WARN(1, "tpm_buf: overflow\n");
+		buf->flags |= TPM_BUF_OVERFLOW;
+		return;
+	}
+
+	memcpy(&buf->data[len], new_data, new_len);
+	head->length = cpu_to_be32(len + new_len);
+}
+
+static inline void tpm_buf_append_u8(struct tpm_buf *buf, const u8 value)
+{
+	tpm_buf_append(buf, &value, 1);
+}
+
+static inline void tpm_buf_append_u16(struct tpm_buf *buf, const u16 value)
+{
+	__be16 value2 = cpu_to_be16(value);
+
+	tpm_buf_append(buf, (u8 *) &value2, 2);
+}
+
+static inline void tpm_buf_append_u32(struct tpm_buf *buf, const u32 value)
+{
+	__be32 value2 = cpu_to_be32(value);
+
+	tpm_buf_append(buf, (u8 *) &value2, 4);
+}
+
 extern struct class *tpm_class;
 extern dev_t tpm_devt;
 extern const struct file_operations tpm_fops;