Complete formal verification is the only known way to guarantee that a system is free of programming errors. We present our experience in performing the formal, machine-checked verification of the seL4 microkernel from an abstract specification down to its C implementation. We assume correctness of compiler, assembly code, and hardware, and we used a unique design approach that fuses formal and operating systems techniques. To our knowledge, this is the first formal proof of functional correctness of a complete, general-purpose operating-system kernel. Functional correctness means here that the implementation always strictly follows our high-level abstract specification of kernel behaviour. This encompasses traditional design and implementation safety properties such as the kernel will never crash, and it will never perform an unsafe operation. It also proves much more: we can predict precisely how the kernel will behave in every possible situation. seL4, a third-generation microkernel of L4 provenance, comprises 8,700 lines of C code and 600 lines of assembler. Its performance is comparable to other high-performance L4 kernels.

We present Flicker, an infrastructure for executing securitysensitive code in complete isolation while trusting as few as 250 lines of additional code. Flicker can also provide meaningful, fine-grained attestation of the code executed (as well as its inputs and outputs) to a remote party. Flicker guarantees these properties even if the BIOS, OS and DMAenabled devices are all malicious. Flicker leverages new commodity processors from AMD and Intel and does not require a new OS or VMM. We demonstrate a full implementation of Flicker on an AMD platform and describe our development environment for simplifying the construction of Flicker-enabled code.

We develop a special-purpose hypervisor called TrustVisor that facilitates the execution of security-sensitive code in isolation from commodity OSes and applications. TrustVisor provides code and execution integrity as well as data secrecy and integrity for protected code, even in the presence of a compromised OS. These strong properties can be attested to a remote verifier. TrustVisor only adds 5306 lines to the TCB (over half of which is for cryptographic operations). TrustVisorimposeslessthan7%overheadinthecommoncase. Thisoverheadislargelytheresult of today’s x86hardware virtualization support. 1

In current commodity systems, applications have no way of limiting their trust in the underlying operating system (OS), leaving them at the complete mercy of an attacker who gains control over the OS. In this work, we describe the design and implementation of Proxos, a system that allows applications to configure their trust in the OS by partitioning the system call interface into trusted and untrusted components. System call routing rules that indicate which system calls are to be handled by the untrusted commodity OS, and which are to be handled by a trusted private OS, are specified by the application developer. We find that rather than defining a new system call interface, routing system calls of an existing interface allows applications currently targeted towards commodity operating systems to isolate their most sensitive components from the commodity OS with only minor source code modifications. We have built a prototype of our system on top of the Xen Virtual Machine Monitor with Linux as the commodity OS. In practice, we find that the system call routing rules are short and simple – on the order of 10’s of lines of code. In addition, applications in Proxos incur only modest performance overhead, with most of the cost resulting from inter-VM context switches. 1

Commodity operating systems entrusted with securing sensitive data are remarkably large and complex, and consequently, frequently prone to compromise. To address this limitation, we introduce a virtual-machine-based system called Overshadow that protects the privacy and integrity of application data, even in the event of a total OS compromise. Overshadow presents an application with a normal view of its resources, but the OS with an encrypted view. This allows the operating system to carry out the complex task of managing an application’s resources, without allowing it to read or modify them. Thus, Overshadow offers a last line of defense for application data. Overshadow builds on multi-shadowing, a novel mechanism that presents different views of “physical ” memory, depending on the context performing the access. This primitive offers an additional dimension of protection beyond the hierarchical protection domains implemented by traditional operating systems and processor architectures. We present the design and implementation of Overshadow and show how its new protection semantics can be integrated with existing systems. Our design has been fully implemented and used to protect a wide range of unmodified legacy applications running on an unmodified Linux operating system. We evaluate the performance of our implementation, demonstrating that this approach is practical.

Virtual machine monitors (VMMs) have been hailed as the basis for an increasing number of reliable or trusted computing systems. The Xen VMM is a relatively small piece of software – a hypervisor – that runs at a lower level than a conventional operating system in order to provide isolation between virtual machines: its size is offered as an argument for its trustworthiness. However, the management of a Xen-based system requires a privileged, fullblown operating system to be included in the trusted computing base (TCB). In this paper, we introduce our work to disaggregate the management virtual machine in a Xen-based system. We begin by analysing the Xen architecture and explaining why the status quo results in a large TCB. We then describe our implementation, which moves the domain builder, the most important privileged component, into a minimal trusted compartment. We illustrate how this approach may be used to implement “trusted virtualisation ” and improve the security of virtual TPM implementations. Finally, we evaluate our approach in terms of the reduction in TCB size, and by performing a security analysis of the disaggregated system. Categories and Subject Descriptors D.4.6 [Operating Systems]: Security and Protection—Information flow controls

Multi-tenant cloud, which usually leases resources in the form of virtual machines, has been commercially available for years. Unfortunately, with the adoption of commodity virtualized infrastructures, software stacks in typical multi-tenant clouds are non-trivially large and complex, and thus are prone to compromise or abuse from adversaries including the cloud operators, which may lead to leakage of security-sensitive data. In this paper, we propose a transparent, backward-compatible approach that protects the privacy and integrity of customers ’ virtual machines on commodity virtualized infrastructures, even facing a total compromise of the virtual machine monitor (VMM) and the management VM. The key of our approach is the separation of the resource management from security protection in the virtualization layer. A tiny security monitor is introduced underneath the commodity VMM using nested virtualization and provides protection to the hosted VMs. As a result, our approach allows virtualization software (e.g., VMM, management VM and tools) to handle complex tasks of managing leased VMs for the cloud, without breaking security of users ’ data inside the VMs. We have implemented a prototype by leveraging commercially-available hardware support for virtualization. The prototype system, called CloudVisor, comprises only 5.5K LOCs and supports the Xen VMM with multiple Linux and Windows as the guest OSes. Performance evaluation shows that CloudVisor incurs moderate slowdown for I/O intensive applications and very small slowdown for other applications.

We propose an architecture that allows code to execute in complete isolation from other software while trusting only a tiny software base that is orders of magnitude smaller than even minimalist virtual machine monitors. Our technique also enables more meaningful attestation than previous proposals, since only measurements of the security-sensitive portions of an application need to be included. We achieve these guarantees by leveraging hardware support provided by commodity processors from AMD and Intel that are shipping today.

System virtualization, which enjoys immense popularity in the enterprise and personal computing spaces, is recently gaining significant interest in the embedded domain. Starting from a comparison of key characteristics of enterprise systems and embedded systems, we will examine the difference in motivation for the use of system virtual machines, and the resulting differences in the requirements for the technology. We find that these differences are quite substantial, and that virtualization is unable to meet the special requirements of embedded systems. Instead, more general operatingsystems technologies are required, which support virtualization as a special case. We argue that high-performance microkernels, specifically L4, are a technology that provides a good match for the requirements of next-generation embedded systems. 1.

In this paper we present the lessons we learned when developing VPFS, a virtual private file system that is based on both a small amount of trusted storage and an untrusted legacy file system residing on the same machine. VPFS’ purpose is to provide secure and reliable storage to highly sensitive applications running on top of a microkernel, which may concurrently execute untrusted software. The confidentiality and integrity guarantees of VPFS do not only apply to file contents, but also to all meta data including integrity of the directory structure. We explored design alternatives that allow us to securely reuse untrusted infrastructure and thereby minimize the complexity that a file-system implementation adds to the trusted computing base. VPFS is split into two isolated components. A small trusted component implements all security-critical functionality, whereas the untrusted part reuses an existing file-system implementation provided by a virtualized legacy operating system that can be untrusted. In our VPFS prototype, alternative configurations of the trusted component comprise only between 4,000 and 4,600 lines of code, which is at least an order of magnitude smaller than existing commodity file-system stacks.