Bluetooth: Host: Fix possible inconsistent access to connection state

An assertion in bt_conn_unref() accesses the connection's state
after decrementing its reference count.
This is not consistent since, if we removed the last reference,
the Bluetooth Host stack may reuse the connection object
before the assertion is checked.
Instead, retrieve the connection property tested by the assertion
before decrementing the counter, as we do for other properties.

Simplify the code path by returning early when we did not remove
the last reference.

Remind that automatic advertiser resumption is deprecated.

Signed-off-by: Christophe Dufaza <[email protected]>

Author: dottspina

subsys/bluetooth/host/conn.c

@@ -1500,38 +1500,57 @@ void bt_conn_unref(struct bt_conn *conn)
 	enum bt_conn_type conn_type;
 	uint8_t conn_role;
 	uint16_t conn_handle;
+	/* Used only if CONFIG_ASSERT and CONFIG_BT_CONN_TX. */
+	__maybe_unused bool conn_tx_is_pending;
 
 	__ASSERT(conn, "Invalid connection reference");
 
-	/* Storing parameters of interest so we don't access the object
-	 * after decrementing its ref-count
+	/* If we're removing the last reference, the connection object will be
+	 * considered freed and possibly re-allocated by the Bluetooth Host stack
+	 * as soon as we decrement the counter.
+	 * To prevent inconsistencies when accessing the connection's state,
+	 * we store its properties of interest before decrementing the ref-count,
+	 * then unset the local pointer.
 	 */
 	conn_type = conn->type;
 	conn_role = conn->role;
 	conn_handle = conn->handle;
-
+#if CONFIG_BT_CONN_TX && CONFIG_ASSERT
+	conn_tx_is_pending = k_work_is_pending(&conn->tx_complete_work);
+#endif
 	old = atomic_dec(&conn->ref);
-	/* Whether we removed the last reference. */
-	deallocated = (old == 1);
-	IF_ENABLED(CONFIG_BT_CONN_TX,
-		   (__ASSERT(!(deallocated && k_work_is_pending(&conn->tx_complete_work)),
-			     "tx_complete_work is pending when conn is deallocated")));
 	conn = NULL;
 
 	LOG_DBG("handle %u ref %ld -> %ld", conn_handle, old, (old - 1));
 
 	__ASSERT(old > 0, "Conn reference counter is 0");
 
-	/* Slot has been freed and can be taken. No guarantees are made on requests
-	 * to claim connection object as only the first claim will be served.
-	 */
-	if (deallocated) {
-		k_sem_give(&pending_recycled_events);
-		k_work_submit(&recycled_work);
+	/* Whether we removed the last reference. */
+	deallocated = (old == 1);
+	if (!deallocated) {
+		return;
 	}
 
+	IF_ENABLED(CONFIG_BT_CONN_TX,
+		   (__ASSERT(!conn_tx_is_pending,
+			     "tx_complete_work is pending when conn is deallocated");))
+
+	/* Notify listeners that a slot has been freed and can be taken.
+	 * No guarantees are made on requests to claim connection object
+	 * as only the first claim will be served.
+	 */
+	k_sem_give(&pending_recycled_events);
+	k_work_submit(&recycled_work);
+
+	/* Use the freed slot to automatically resume LE peripheral advertising.
+	 *
+	 * This behavior is deprecated:
+	 * - 8cfad44: Bluetooth: Deprecate adv auto-resume
+	 * - #72567: Bluetooth: Advertising resume functionality is broken
+	 * - Migration guide to Zephyr v4.0.0, Automatic advertiser resumption is deprecated
+	 */
 	if (IS_ENABLED(CONFIG_BT_PERIPHERAL) && conn_type == BT_CONN_TYPE_LE &&
-	    conn_role == BT_CONN_ROLE_PERIPHERAL && deallocated) {
+	    conn_role == BT_CONN_ROLE_PERIPHERAL) {
 		bt_le_adv_resume();
 	}
 }