[PATCH v2 1/2] common: machine_id: support deriving app specific UUIDs

["Uwe Kleine-König" <uwe@kleine-koenig.org>]

From: Ahmad Fatoum <a.fatoum@pengutronix.de>

libsystemd provides a sd_id128_get_machine_app_specific() function that
allows deriving an application specific UUID without directly leaking
the machine ID.

Let's provide an equivalent for barebox that will be used in a following
commit to generate a stable MAC instead of randomizing it.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
---
 common/machine_id.c  | 84 ++++++++++++++++++++++++++++++++++++++++++--
 include/linux/uuid.h |  8 +++++
 include/machine_id.h | 11 ++++++
 3 files changed, 101 insertions(+), 2 deletions(-)

diff --git a/common/machine_id.c b/common/machine_id.c
index 8c273b934989..edc8ad0ac156 100644
--- a/common/machine_id.c
+++ b/common/machine_id.c
@@ -13,17 +13,96 @@
 
 #define MACHINE_ID_LENGTH 32
 
+static bool __machine_id_initialized;
 static void *__machine_id_hashable;
 static size_t __machine_id_hashable_length;
 
-
+/**
+ * machine_id_set_hashable - Provide per-board unique data
+ * @hashable: Buffer
+ * @len: size of buffer
+ *
+ * The data supplied to the last call of this function prior to
+ * late_initcall will be hashed and stored into global.machine_id,
+ * which can be later used for fixup into the kernel command line
+ * or for deriving application specific unique IDs via
+ * machine_id_get_app_specific().
+ */
 void machine_id_set_hashable(const void *hashable, size_t len)
 {
-
 	__machine_id_hashable = xmemdup(hashable, len);
 	__machine_id_hashable_length = len;
 }
 
+/**
+ * machine_id_get_app_specific - Generates an application-specific UUID
+ * @result: UUID output of the function
+ * @...: pairs of (const void *, size_t) arguments of data to factor
+ * into the UUID followed by a NULL sentinel value.
+ *
+ * Combines the machine ID with the application specific varargs data
+ * to arrive at an application-specific and board-specific UUID that is
+ * stable and unique.
+ *
+ * The function returns 0 if a UUID was successfully written into @result
+ * and a negative error code otherwise.
+ */
+int machine_id_get_app_specific(uuid_t *result, ...)
+{
+	static u8 hmac[SHA256_DIGEST_SIZE];
+	const void *data;
+	size_t size;
+	va_list args;
+	struct digest *d;
+	int ret;
+
+	if (!__machine_id_initialized)
+		return -ENODATA;
+
+	d = digest_alloc("hmac(sha256)");
+	if (!d)
+		return -ENOSYS;
+
+	ret = digest_set_key(d, __machine_id_hashable, __machine_id_hashable_length);
+	if (ret)
+		goto out;
+
+	ret = digest_init(d);
+	if (ret)
+		goto out;
+
+	ret = -ENODATA;
+
+	va_start(args, result);
+
+	while ((data = va_arg(args, const void *))) {
+		size = va_arg(args, size_t);
+
+		ret = digest_update(d, data, size);
+		if (ret)
+			break;
+	}
+
+	va_end(args);
+
+	if (ret)
+		goto out;
+
+	ret = digest_final(d, hmac);
+	if (ret)
+		goto out;
+
+	/* Take only the first half. */
+	memcpy(result, hmac, min(sizeof(hmac), sizeof(*result)));
+
+	uuid_make_v4(result);
+
+out:
+	digest_free(d);
+
+	return ret;
+}
+
 static int machine_id_set_globalvar(void)
 {
 	struct digest *digest = NULL;
@@ -61,6 +140,7 @@ static int machine_id_set_globalvar(void)
 	env_machine_id = basprintf("%.*s", MACHINE_ID_LENGTH, hex_machine_id);
 	globalvar_add_simple("machine_id", env_machine_id);
 	free(env_machine_id);
+	__machine_id_initialized = true;
 
 out:
 	digest_free(digest);
diff --git a/include/linux/uuid.h b/include/linux/uuid.h
index 6b1a3efa1e0b..1e4ffb343452 100644
--- a/include/linux/uuid.h
+++ b/include/linux/uuid.h
@@ -107,6 +107,14 @@ extern const u8 uuid_index[16];
 int guid_parse(const char *uuid, guid_t *u);
 int uuid_parse(const char *uuid, uuid_t *u);
 
+static inline void uuid_make_v4(uuid_t *u) {
+	/* Set UUID version to 4 --- truly random generation */
+	u->b[6] = (u->b[6] & 0x0F) | 0x40;
+
+	/* Set the UUID variant to DCE */
+	u->b[8] = (u->b[8] & 0x3F) | 0x80;
+}
+
 /* MEI UUID type, don't use anywhere else */
 #include <uapi/linux/uuid.h>
 
diff --git a/include/machine_id.h b/include/machine_id.h
index e30bbada1acd..9245a5412770 100644
--- a/include/machine_id.h
+++ b/include/machine_id.h
@@ -3,9 +3,13 @@
 #ifndef __MACHINE_ID_H__
 #define __MACHINE_ID_H__
 
+#include <linux/types.h>
+#include <linux/uuid.h>
+
 #if IS_ENABLED(CONFIG_MACHINE_ID)
 
 void machine_id_set_hashable(const void *hashable, size_t len);
+int machine_id_get_app_specific(uuid_t *result, ...) __attribute__((__sentinel__));
 
 #else
 
@@ -13,6 +17,13 @@ static inline void machine_id_set_hashable(const void *hashable, size_t len)
 {
 }
 
+static inline int machine_id_get_app_specific(uuid_t *result, ...)
+	__attribute__((__sentinel__));
+static inline int machine_id_get_app_specific(uuid_t *result, ...)
+{
+	return -ENOSYS;
+}
+
 #endif /* CONFIG_MACHINE_ID */
 
 #endif  /* __MACHINE_ID_H__ */
-- 
2.42.0.586.gbc5204569f7d.dirty