Architectural support for copy and tamper resistant software

TLDR

Attempts to create tamper‑resistant software via code transformations have failed to produce reliable software‑only methods, and the design of the proposed machine highlights trade‑offs among security, efficiency, and flexibility. This paper studies the hardware implementation of a form of execute‑only memory (XOM) that allows instructions stored in memory to be executed but not otherwise manipulated. The authors realize XOM with a compartmentalized machine that encrypts all external data, enforces private keys, private memory, and cache‑miss traps, and uses hardware‑accelerated symmetric ciphers to explore trade‑offs between security, efficiency, and flexibility. Analysis shows that despite a performance penalty, a normal multi‑tasking machine can run almost all applications in XOM mode.

Abstract

Although there have been attempts to develop code transformations that yield tamper-resistant software, no reliable software-only methods are know. This paper studies the hardware implementation of a form of execute-only memory (XOM) that allows instructions stored in memory to be executed but not otherwise manipulated. To support XOM code we use a machine that supports internal compartments---a process in one compartment cannot read data from another compartment. All data that leaves the machine is encrypted, since we assume external memory is not secure. The design of this machine poses some interesting trade-offs between security, efficiency, and flexibility. We explore some of the potential security issues as one pushes the machine to become more efficient and flexible. Although security carries a performance penalty, our analysis indicates that it is possible to create a normal multi-tasking machine where nearly all applications can be run in XOM mode. While a virtual XOM machine is possible, the underlying hardware needs to support a unique private key, private memory, and traps on cache misses. For efficient operation, hardware assist to provide fast symmetric ciphers is also required.