A security kernel is responsible for enforcing a security policy. It is a strict implementation of a reference monitor mechanism.
The architecture of a kernel operating system is typically layered, and the kernel should be at the lowest and most primitive level.
It is a small portion of the operating system through which all references to information and all changes to authorizations must pass. In theory, the kernel implements access control and information flow control between implemented objects according to the security policy.
To be secure, the kernel must meet three basic conditions:
completeness (all accesses to information must go through the kernel),
isolation (the kernel itself must be protected from any type of unauthorized access), and verifiability (the kernel must be proven to meet design specifications).
The reference monitor, as noted previously, is an abstraction, but there may be a reference validator, which usually runs inside the security kernel and is responsible for performing security access checks on objects, manipulating privileges, and generating any resulting security audit messages.
A term associated with security kernels and the reference monitor is the trusted computing base (TCB). The TCB is the portion of a computer system that contains all elements of the system responsible for supporting the security policy and the isolation of objects. The security capabilities of products for use in the TCB can be verified through various evaluation criteria, such as the earlier Trusted Computer System Evaluation Criteria (TCSEC) and the current Common Criteria standard.
Many of these security terms—reference monitor, security kernel, TCB—are defined loosely by vendors for purposes of marketing literature. Thus, it is necessary for security professionals to read the small print and between the lines to fully understand what the vendor is offering in regard to security features.
TIP FOR THE EXAM: The terms Security Kernel and Reference monitor are synonymous but at different levels. As it was explained by Diego:
While the Reference monitor is the concept, the Security kernel is the implementation of such concept (via hardware, software and firmware means).
The two terms are the same thing, but on different levels: one is conceptual, one is “technical”
The following are incorrect answers: Confidentiality, Integrity, and Availability Policy, mechanism, and assurance Isolation, layering, and abstraction
Reference(s) used for this question: Hernandez CISSP, Steven (2012-12-21). Official (ISC)2 Guide to the CISSP CBK, Third Edition ((ISC)2 Press) (Kindle Locations 13858-13875). Auerbach Publications. Kindle Edition.