SLIC: An Extensibility System for Commodity Operating Systems

Modern commodity operating systems are large and complex systems developed over many years by large teams of programmers, containing hundreds of thousands of lines of code. Consequently, it is extremely difficult to add significant new functionality to these systems. In response to this problem, a number of recent research projects have explored novel operating system architectures to support untrusted extensions, including SPIN, VINO, Exokernel, and Fluke. Unfortunately, these architectures require substantial implementation effort and are not generally available in commodity systems. In contrast, by leveraging the technique of interposition, we have designed and implemented a prototype extension system called SLIC which requires only trivial operating system changes. SLIC efficiently inserts trusted extension code into commodity operating systems, enabling a large class of trusted extensions for existing commodity operating systems such as Solaris and Linux, while retaining full compatibility with existing application binaries. By interposing trusted extensions on existing kernel interfaces, our solution enables extensions which are protected from malicious applications, are enforced upon uncooperative applications, are composable with extensions from other third-party sources, and can be developed at the user-level using state-of-the-art development tools. We have used SLIC to implement and demonstrate a number of useful operating system extensions, including a patch to fix a security hole described in a CERT advisory, a simple encryption file system, and a restricted execution environment for arbitrary untrusted binaries. Performance measurements of the SLIC prototype demonstrate a one-time installation cost of 2-8 µsec and a per-extension invocation overhead commensurate with a procedure call.