Bluetooth: host: Optionally disable GATT sec re-establishment

Some centrals deal poorly with receiving a security request immediately
after reconnection whenever reconnecting with characteristics that are
notifiable or indicatable and requiring security. In particular,
Android 9 and earlier devices may lose bond information when this
happens, some Microsoft Surface devices will enter an invalid state
and, on top of that, Apple's Bluetooth Low Energy guidelines explicitly
discourage this behavior.

In order to allow interoperability with those devices, make the GATT
automatic security request sending as a peripheral optional by
introducing a new Kconfig option, BT_GATT_AUTO_SEC_REQ.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>

subsys/bluetooth/host/Kconfig.gatt

@@ -55,6 +55,24 @@ config BT_EATT_SEC_LEVEL
 
 endif # BT_EATT
 
+config BT_GATT_AUTO_SEC_REQ
+	bool "Automatic security re-establishment request as a peripheral"
+	default y
+	depends on BT_SMP
+	help
+	  This option requests security re-establishment automatically whenever
+	  a reconnection to a GATT client is established and
+	  there are notifiable or indicatable characteristics for which security
+	  is required. This configuration option only applies to the peripheral
+	  role, which sends a Security Request for this purpose.
+	  When disabled, it is the application's responsibility to call
+	  bt_conn_set_security() to re-establish security prior to sending any
+	  notifications or indications on the characteristics that require
+	  security (unless the central does that first).
+	  This option has been introduced to avoid interoperability issues with
+	  commercially available central devices that react negatively to
+	  receiving a Security Request immediately after reconnection.
+
 config BT_GATT_SERVICE_CHANGED
 	bool "GATT Service Changed support"
 	default y

subsys/bluetooth/host/gatt.c

@@ -4861,6 +4861,8 @@ void bt_gatt_connected(struct bt_conn *conn)
 	 * enabling encryption will fail.
 	 */
 	if (IS_ENABLED(CONFIG_BT_SMP) &&
+	    (conn->role == BT_HCI_ROLE_MASTER ||
+	     IS_ENABLED(CONFIG_BT_GATT_AUTO_SEC_REQ)) &&
 	    bt_conn_get_security(conn) < data.sec) {
 		int err = bt_conn_set_security(conn, data.sec);