Improving Operating System Security, Reliability, and Performance through Intra-Unikernel Isolation, Asynchronous Out-of-kernel IPC, and Advanced System Servers

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Date

2023-03-28

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Virginia Tech

Abstract

Computer systems are vulnerable to security exploits, and the security of the operating system (OS) is crucial as it is often a trusted entity that applications rely on. Traditional OSs have a monolithic design where all components are executed in a single privilege layer, but this design is increasingly inadequate as OS code sizes have become larger and expose a large attack surface. Microkernel OSs and multiserver OSs improve security and reliability through isolation, but they come at a performance cost due to crossing privilege layers through IPCs, system calls, and mode switches. Library OSs, on the other hand, implement kernel components as libraries which avoids crossing privilege layers in performance-critical paths and thereby improves performance. Unikernels are a specialized form of library OSs that consist of a single application compiled with the necessary kernel components, and execute in a single address space, usually atop a hypervisor for strong isolation. Unikernels have recently gained popularity in various application domains due to their better performance and security.

Although unikernels offer strong isolation between each instance due to virtualization, there is no isolation within a unikernel. Since the model eliminates the traditional separation between kernel and user parts of the address space, the subversion of a kernel or application component will result in the subversion of the entire unikernel. Thus, a unikernel must be viewed as a single unit of trust, reducing security. The dissertation's first contribution is intra-unikernel isolation: we use Intel's Memory Protection Keys (MPK) primitive to provide per-thread permission control over groups of virtual memory pages within a unikernel's single address space, allowing different areas of the address space to be isolated from each other. We implement our mechanisms in RustyHermit, a unikernel written in Rust. Our evaluations show that the mechanisms have low overhead and retain unikernel's low system call latency property: 0.6% slowdown on applications including memory/compute intensive benchmarks as well as micro-benchmarks.

Multiserver OS, a type of microkernel OS, has high parallelism potential due to its inherent compartmentalization. However, the model suffers from inferior performance. This is due to inter-process communication (IPC) client-server crossings that require context switches for single-core systems, which are more expensive than traditional system calls; on multi-core systems (now ubiquitous), they have poor resource utilization. The dissertation's second contribution is Aoki, a new approach to IPC design for microkernel OSs. Aoki incorporates non-blocking concurrency techniques to eliminate in-kernel blocking synchronization which causes performance challenges for state-of-the-art microkernels. Aoki's non-blocking (i.e., lock-free and wait-free) IPC design not only improves performance and scalability, but also enhances reliability by preventing thread starvation. In a multiserver OS setting, the design also enables the reconnection of stateful servers after failure without loss of IPC states. Aoki solves two problems that have plagued previous microkernel IPC designs: reducing excessive transitions between user and kernel modes and enabling efficient recovery from failures. We implement Aoki in the state-of-the-art seL4 microkernel. Results from our experiments show that Aoki outperforms the baseline seL4 in both fastpath IPC and cross-core IPC, with improvements of 2.4x and 20x, respectively.

The Aoki IPC design enables the design of system servers for multiserver OSs with higher performance and reliability. The dissertation's third and final contribution is the design of a fault-tolerant storage server and a copy-free file system server. We build both servers using NetBSD OS's rumprun unikernel, which provides robust isolation through hardware virtualization, and is capable of handling a wide range of storage devices including NVMe. Both servers communicate with client applications using Aoki's IPC design, which yields scalable IPC. In the case of the storage server, the IPC also enables the server to transparently recover from server failures and reconnect to client applications, with no loss of IPC state and no significant overhead. In the copy-free file system server's design, applications grant the server direct memory access to file I/O data buffers for high performance. The performance problems solved in the server designs have challenged all prior multiserver/microkernel OSs. Our evaluations show that both servers have a performance comparable to Linux and the rumprun baseline.

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Keywords

Operating Systems, Microkernels, Unikernels, IPC, Security, Failure Recovery

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