Device drivers commonly execute in the kernel to achieve high performance and easy access to kernel services. However, this comes at the price of decreased reliability and increased programming difficulty. Driver programmers are unable to use user-mode development tools and must instead use cumbersome kernel tools. Faults in kernel drivers can cause the entire operating system to crash. Usermode drivers have long been seen as a solution to this problem, but suffer from either poor performance or new interfaces that require a rewrite of existing drivers. This paper introduces the Microdrivers architecture that achieves high performance and compatibility by leaving critical path code in the kernel and moving the rest of the driver code to a user-mode process. This allows data-handling operations critical to I/O performance to run at full speed, while management operations such as initialization and configuration run at reduced speed in userlevel. To achieve compatibility, we present DriverSlicer, a tool that splits existing kernel drivers into a kernel-level component and a user-level component using a small number of programmer annotations. Experiments show that as much as 65 % of driver code can be removed from the kernel without affecting common-case performance, and that only 1-6 percent of the code requires annotations.

Device drivers on commodity operating systems execute with kernel privilege and have unfettered access to kernel data structures. Several recent attacks demonstrate that such poor isolation exposes kernel data to exploits against vulnerable device drivers, for example through buffer overruns in packet processing code. Prior architectures to isolate kernel data from driver code either sacrifice performance, execute too much driver code with kernel privilege, or are incompatible with commodity operating systems. In this paper, we present the design, implementation and evaluation of a novel security architecture that better isolates kernel data from device drivers without sacrificing performance or compatibility. In this architecture, a device driver is partitioned into a small, trusted kernel-mode component and an untrusted user-mode component. The kernel-mode component contains privileged and performance-critical code. It communicates via RPC with the user-mode component which contains the rest of the driver code. A RPC monitor mediates all control and data transfers between the kernel- and user-mode components. In particular, it verifies that all data transfers from the untrusted user-mode component to the kernel-mode component preserve kernel data structure integrity. We also present a runtime technique to automatically infer such integrity specifications. Our experiments with a Linux implementation of this architecture show that it can prevent compromised device drivers from affecting the integrity of kernel data and do so without impacting common-case performance. 1.

Abstract Device drivers commonly execute in the kernel to achieve high per-formance and easy access to kernel services. However, this comes