A common approach used to add security to a system is to intercept service requests or to otherwise interpose a layer of security code between all applications and the operating system (e.g., Kernel Hypervisors , SPIN ), or between particular applications or sets of applications (e.g., L3/L4 , Lava , KeySAFE ). This may be done in capability systems or non-capability systems, and when applied to an operating system the security layer may lie within the operating system itself (as in Spring ) or in a component outside of the operating system to which all requests are redirected (as in Janus ).
However, this approach has some serious limitations. In order to add security by intercepting requests, the existing functional interface must expose all abstractions and information flows that the security policy wishes to control. To avoid maintaining redundant state in the access control layer, the functional interface must ensure that all security-relevant attributes are either directly available as parameters or easily derived from parameters. A policy that requires the use of some internal state of the object manager as an input to the decision can not be implemented without either changing the manager to export the state or, if possible, replicating the state management in the enforcer itself. The level of abstraction provided by the interface may be inappropriate or may cause difficulties in guaranteeing uniqueness or atomicity. For example, typical name-based calls suffer from issues of aliasing, multi-component lookups, and preserving the tranquility of the name-to-object mapping from the time-of-check to the time-of-use. Finally, this approach is limited in that the security layer can only affect the operation of the system as requests pass through it. Hence, it is often impossible for the system to reflect subsequent changes to the security policy, in particular, the revocation of migrated permissions.
As was the case with capabilities, implementing access control within a security layer is a good approach when these disadvantages can be avoided through the use of other mechanisms. However, it is important to recognize that other mechanisms are necessary, often mechanisms that are more invasive than intercepting requests, in order to provide any degree of flexibility in supporting security policies.