tpm: add tpm_get_random()

Add a function to obtain random data from the TPM.

Signed-off-by: André Draszik <adraszik@tycoint.com>
Added commit message, add cast to min()
Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Simon Glass <sjg@chromium.org>

include/tpm.h

@@ -651,4 +651,16 @@ uint32_t tpm_flush_specific(uint32_t key_handle, uint32_t resource_type);
 uint32_t tpm_find_key_sha1(const uint8_t auth[20], const uint8_t
 			   pubkey_digest[20], uint32_t *handle);
 #endif /* CONFIG_TPM_LOAD_KEY_BY_SHA1 */
+
+/**
+ * Read random bytes from the TPM RNG. The implementation deals with the fact
+ * that the TPM may legally return fewer bytes than requested by retrying
+ * until @p count bytes have been received.
+ *
+ * @param data		output buffer for the random bytes
+ * @param count		size of output buffer
+ * @return return code of the operation
+ */
+uint32_t tpm_get_random(void *data, uint32_t count);
+
 #endif /* __TPM_H */

lib/tpm.c

@@ -1049,3 +1049,46 @@ uint32_t tpm_find_key_sha1(const uint8_t auth[20], const uint8_t
 #endif /* CONFIG_TPM_LOAD_KEY_BY_SHA1 */
 
 #endif /* CONFIG_TPM_AUTH_SESSIONS */
+
+uint32_t tpm_get_random(void *data, uint32_t count)
+{
+	const uint8_t command[14] = {
+		0x0, 0xc1,		/* TPM_TAG */
+		0x0, 0x0, 0x0, 0xe,	/* parameter size */
+		0x0, 0x0, 0x0, 0x46,	/* TPM_COMMAND_CODE */
+	};
+	const size_t length_offset = 10;
+	const size_t data_size_offset = 10;
+	const size_t data_offset = 14;
+	uint8_t buf[COMMAND_BUFFER_SIZE], response[COMMAND_BUFFER_SIZE];
+	size_t response_length = sizeof(response);
+	uint32_t data_size;
+	uint8_t *out = data;
+
+	while (count > 0) {
+		uint32_t this_bytes = min((size_t)count,
+					  sizeof (response) - data_offset);
+		uint32_t err;
+
+		if (pack_byte_string(buf, sizeof(buf), "sd",
+				     0, command, sizeof(command),
+				     length_offset, this_bytes))
+			return TPM_LIB_ERROR;
+		err = tpm_sendrecv_command(buf, response, &response_length);
+		if (err)
+			return err;
+		if (unpack_byte_string(response, response_length, "d",
+				       data_size_offset, &data_size))
+			return TPM_LIB_ERROR;
+		if (data_size > count)
+			return TPM_LIB_ERROR;
+		if (unpack_byte_string(response, response_length, "s",
+				       data_offset, out, data_size))
+			return TPM_LIB_ERROR;
+
+		count -= data_size;
+		out += data_size;
+	}
+
+	return 0;
+}