1. A computer-implemented method, comprising: receiving, by a virtual machine (VM) executing on a computing device, a custom security type label (STL) and a custom security policy associated with the custom STL, the custom STL and associated custom security policy applied to one or more multi-tenant applications executed by the VM; receiving, by the VM, a request to initialize an application on the VM, the request identifying the custom STL as an STL to apply to the application; assigning, by the VM, a local UID maintained by the VM to the application; recording, by the VM, a mapping of the assigned local UID to the custom STL; assigning, by the VM, the custom STL to files of the application; and assigning, by the VM, the custom STL to a running process of the application.