[PATCH BlueZ 1/2] gatt: allow GATT server to dicate CCC permissions

[Dagan Martinez <dmartinez@starry.com>]

Allow a GATT server to impose permissions/restrictions on a CCC by
setting additional `X-asynchronous` permissions on its associated
characteristic.

This allows a developer to require encryption/authentication in order
for a GATT client to subscribe to server-initiated updates.

Test procedure:
Attempt to read/write with a "low" security level on an unprotected CCC
using gatttool, and succeed
Attempt to READ with a "low" security level on an protected CCC
using gatttool, and succeed
Attempt to WRITE with a "low" security level on an protected CCC
using gatttool, and fail
Attempt to read/write while paired on a protected CCC using
`bluetoothctl`, and succeed
---
 src/gatt-database.c    | 29 +++++++++++++++++++++++++----
 src/shared/att-types.h |  4 ++++
 2 files changed, 29 insertions(+), 4 deletions(-)

diff --git a/src/gatt-database.c b/src/gatt-database.c
index 1f7ce5f02..bb65656bd 100644
--- a/src/gatt-database.c
+++ b/src/gatt-database.c
@@ -1044,6 +1044,7 @@ service_add_ccc(struct gatt_db_attribute *service,
 				struct btd_gatt_database *database,
 				btd_gatt_database_ccc_write_t write_callback,
 				void *user_data,
+				uint32_t parent_permissions,
 				btd_gatt_database_destroy_t destroy)
 {
 	struct gatt_db_attribute *ccc;
@@ -1052,9 +1053,23 @@ service_add_ccc(struct gatt_db_attribute *service,
 
 	ccc_cb = new0(struct ccc_cb_data, 1);
 
+	/*
+	 * Provide a way for the permissions on a characteristic to dictate
+	 * the permissions on the CCC
+	 */
+	uint32_t permissions = BT_ATT_PERM_READ | BT_ATT_PERM_WRITE;
+
+	if (parent_permissions & BT_ATT_PERM_ASYNCHRONOUS_ENCRYPT)
+		permissions |= BT_ATT_PERM_WRITE_ENCRYPT;
+
+	if (parent_permissions & BT_ATT_PERM_ASYNCHRONOUS_AUTHEN)
+		permissions |= BT_ATT_PERM_WRITE_AUTHEN;
+
+	if (parent_permissions & BT_ATT_PERM_ASYNCHRONOUS_SECURE)
+		permissions |= BT_ATT_PERM_WRITE_SECURE;
+
 	bt_uuid16_create(&uuid, GATT_CLIENT_CHARAC_CFG_UUID);
-	ccc = gatt_db_service_add_descriptor(service, &uuid,
-				BT_ATT_PERM_READ | BT_ATT_PERM_WRITE,
+	ccc = gatt_db_service_add_descriptor(service, &uuid, permissions,
 				gatt_ccc_read_cb, gatt_ccc_write_cb, database);
 	if (!ccc) {
 		error("Failed to create CCC entry in database");
@@ -1211,7 +1226,7 @@ static void populate_gatt_service(struct btd_gatt_database *database)
 				NULL, NULL, database);
 
 	database->svc_chngd_ccc = service_add_ccc(service, database, NULL, NULL,
-									NULL);
+									0, NULL);
 
 	bt_uuid16_create(&uuid, GATT_CHARAC_CLI_FEAT);
 	database->cli_feat = gatt_db_service_add_characteristic(service,
@@ -1674,6 +1689,12 @@ static bool parse_chrc_flags(DBusMessageIter *array, uint8_t *props,
 			*perm |= BT_ATT_PERM_WRITE | BT_ATT_PERM_WRITE_SECURE;
 		} else if (!strcmp("authorize", flag)) {
 			*req_prep_authorization = true;
+		} else if (!strcmp("encrypt-asynchronous", flag)) {
+			*perm |= BT_ATT_PERM_ASYNCHRONOUS_ENCRYPT;
+		} else if (!strcmp("encrypt-authenticated-asynchronous", flag)) {
+			*perm |= BT_ATT_PERM_ASYNCHRONOUS_AUTHEN;
+		} else if (!strcmp("secure-asynchronous", flag)) {
+			*perm |= BT_ATT_PERM_ASYNCHRONOUS_SECURE;
 		} else {
 			error("Invalid characteristic flag: %s", flag);
 			return false;
@@ -2773,7 +2794,7 @@ static bool database_add_ccc(struct external_service *service,
 		return true;
 
 	chrc->ccc = service_add_ccc(service->attrib, service->app->database,
-						ccc_write_cb, chrc, NULL);
+						ccc_write_cb, chrc, chrc->perm, NULL);
 	if (!chrc->ccc) {
 		error("Failed to create CCC entry for characteristic");
 		return false;
diff --git a/src/shared/att-types.h b/src/shared/att-types.h
index a08b24155..554441aca 100644
--- a/src/shared/att-types.h
+++ b/src/shared/att-types.h
@@ -137,6 +137,10 @@ struct bt_att_pdu_error_rsp {
 					BT_ATT_PERM_WRITE_AUTHEN | \
 					BT_ATT_PERM_WRITE_ENCRYPT | \
 					BT_ATT_PERM_WRITE_SECURE)
+/* Permissions to be applied to the CCC */
+#define BT_ATT_PERM_ASYNCHRONOUS_ENCRYPT 0x0400
+#define BT_ATT_PERM_ASYNCHRONOUS_AUTHEN 0x0800
+#define BT_ATT_PERM_ASYNCHRONOUS_SECURE 0x1000
 
 /* GATT Characteristic Properties Bitfield values */
 #define BT_GATT_CHRC_PROP_BROADCAST			0x01
-- 
2.31.1