CACM: A decade of OS access-control extensibility

Operating-system access control technology has undergone a remarkable transformation over the last fifteen years as appliance, embedded, and mobile device vendors transitioned from dedicated “embedded operating systems” to general-purpose ones — often based on open-source UNIX and Linux variants. Device vendors look to upstream operating system authors to provide the critical low-level software foundations for their products: network stacks, UI frameworks, application frameworks, etc. Increasingly, those expectations include security functionality — initially, features to prevent device bricking, but also to constrain potentially malicious code from third-party applications, which engages features from digital signatures to access control and sandboxing.

In a February 2013 Communications of the ACM article, A decade of OS access-control extensibility, I reflect on the central role of kernel access-control extensibility frameworks in supporting security localisation, the adaptation of operating-system security models to site-local or product-specific requirements. Similar to device driver stacks of the virtual file system (VFS), the goal is to allow third-party developers or integrators to extend base operating system security models without being exposed to unstable programming interfaces or the risks associated with less integrated techniques such as system-call interposition.

Case in point is the TrustedBSD MAC Framework, developed and deployed over the 2000s with support from DARPA and the US Navy, in collaboration with several industrial partners. In the article, I consider our original motivations, context, and design principles, but also track the transition process, which relied heavily on open source methodology and community, to a number of widely used products, including the open-source FreeBSD operating system, Apple’s Mac OS X and iOS operating systems, Juniper’s Junos router operating system, and nCircle’s IP360 product. I draw conclusions on things we got right (common infrastructure spanning models; tight integration with OS concurrency model) and wrong (omitting OS privilege model extension; not providing an application author identity model).

Throughout, the diversity of approaches and models suggests an argument for domain-specific policy models that respond to local tradeoffs between performance, functionality, complexity, and security, rather than a single policy model to rule them all. I also emphasise the importance of planning for long-term sustainability for research products — critical to adoption, especially via open source, but also frequently overlooked in academic research.

An open-access (and slightly extended) version of the article can be found on ACM Queue.

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