Bluetooth: Refcnt drop must be placed last in hci_conn_unlink

If hci_conn_put(conn->parent) reduces conn->parent's reference count to
zero, it can immediately deallocate conn->parent. At the same time,
conn->link->list has its head in conn->parent, causing use-after-free
problems in the latter list_del_rcu(&conn->link->list).

This problem can be easily solved by reordering the two operations,
i.e., first performing the list removal with list_del_rcu and then
decreasing the refcnt with hci_conn_put.
Reported-by: Luiz Augusto von Dentz <luiz.dentz@gmail.com>
Closes: https://lore.kernel.org/linux-bluetooth/CABBYNZ+1kce8_RJrLNOXd_8=Mdpb=2bx4Nto-hFORk=qiOkoCg@mail.gmail.com/
Fixes: 06149746 ("Bluetooth: hci_conn: Add support for linking multiple hcon")
Signed-off-by: Ruihan Li <lrh2000@pku.edu.cn>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>

net/bluetooth/hci_conn.c

@@ -1102,12 +1102,12 @@ static void hci_conn_unlink(struct hci_conn *conn)
 	if (!conn->link)
 		return;
 
-	hci_conn_put(conn->parent);
-	conn->parent = NULL;
-
 	list_del_rcu(&conn->link->list);
 	synchronize_rcu();
 
+	hci_conn_put(conn->parent);
+	conn->parent = NULL;
+
 	kfree(conn->link);
 	conn->link = NULL;
 }