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Guest Editorial: Thoughts on secure operating systems
2004-07-19 Article Rating:  / 1
Foreword -- This guest editorial by Victor Yodaiken looks at several operating system (OS) certifications that have recently been used as ammunition against Linux by real-time OS vendors targeting the high-security and military markets.
It also debunks several emotional and inflamatory arguments impugning Linux security. Yodaiken is CEO of FSMLabs, which markets RTLinux, a patented real-time, dual-kernel operating system in which Linux runs as the idle task. FSMLabs in April enhanced RTLinux with support for VME, a durable bus popular in mil/aero applications. Remarks attributed to Gene Spafford and Cynthia Irvine by the EE Times and a marketing offensive by Green Hills against Linux don't provide an accurate picture of software security issues for operating systems and, in fact, add to the confusion. In what follows, I want to try to move the discussion to a less emotional and more balanced basis. One of my points is that security certifications have serious limitations and costs. That's not to say that certifications are bad or useless -- far from it. But certification is not a cure-all, or without problems, and people need to be able to distinguish between marketing and actual engineering. 1. Professor Spafford's complaint about the "provenance" of code in Linux's open development model is unfounded. There is no assurance that any software development effort is free from people who have bad intent or who just write lousy software. The US government's highest security agencies have discovered spies working at the most trusted levels – does anyone realistically expect that software companies will adopt more rigorous screening than the CIA? In any case, it's not clear that it is easier to get code into Linux than it is to get code into other operating systems. In fact, because Linux code is developed on an open model and is tracked by a comprehensive source control system, it may be relatively harder to smuggle malicious code into Linux. In any case, "provenance" is a side issue, one that is easily turned into cheap fear-mongering and xenophobia (see Green Hills's Linux page). For the same reason, I very much dislike the terminology "subversive code," which is an emotionally charged substitute for the standard term "malicious code." The real issues are whether software is designed and tested for security, and whether the development process assures certain levels of quality. 2. The state of the art should not be over-estimated. When, according to the EE Times, Professor Irvine contrasts Linux with "'high-assurance' operating systems with the smarts to prove that subverting code doesn't exist," we should understand that there are no, none, zero, existing operating systems that can prove that they don't contain malicious code or other security flaws. The Common Criteria is a "best practices" standard developed by agencies of several governments (in the US, the NSA and NIAP), but the standard is relatively new and untested. It seems as if it should produce more secure software, but we don't know for sure. Experts like Bruce Schneir argue that standards themselves are of limited use (see Bruce's Crypto-Gram Newsletter dated May 15, 2001). When Linux is criticized for not having Common Criteria certifications, we should note that there are no operating systems certified at the highest level of the Common Criteria, and not even any widely accepted Common Criteria specifications for the security of embedded operating systems. In fact, there is considerable disagreement among researchers over best methods, a shortage of empirical data, and limits to what can be verified at the highest level. 3.The Common Criteria standard defines "evaluation assurance levels" (EAL) from 1 to 7, with 7 being the highest. These levels are being used in a grossly misleading manner. The EAL is just a measure of the level of effort and rigor put into proving that a software program satisfies a security specification. If the specification does not identify the actual threats to the system, the software is not secure, no matter what the level of evaluation assurance. A rigorous proof that the Titanic is unsinkable in the Caribbean Sea is no comfort on a voyage through the North Atlantic iceberg belt. Flooding attacks that can cause a real-time operating system to have scheduling delays are among the threats we consider in our design and test process at FSMLabs. GreenHills is currently embarked on an EAL6 (not 7) effort against a specification that does not have a single real-time requirement. Proof that an operating system meets a specification which does not address the real-time flooding threat provides no security assurance against that threat. One of the virtues of the Common Criteria is that it repeatedly warns against claims like "my software is more secure than yours." The Common Criteria framework encourages people to define "more secure" against the actual type and role of the software and a clear threat analysis. It would be unfortunate indeed if the fundamental message of the Common Criteria was ignored, and it was used only as a tool for frightening people into purchasing or not purchasing software based on meaningless buzzwords. (See this article by Jonathan S. Shapiro, Ph.D, for some similar comments and a very different point of view.) 4. The higher EALs are not ironclad. Even EAL7 only requires a "semi-formal" specification of the low level implementation, so there is no mathematical proof that the low level specification matches the implementation. In any case, the differences between "formal," "semi-formal," and "informal" are not defined in any rigorous manner, and for good reason. Formal specification of complex software is a long way from being a solved problem. There are no existing EAL7 certified operating systems – not a single one (except maybe hidden in some NSA lab). Furthermore, at the top EAL7 level, there are significant limitations on the practicability of meeting the requirements, partly due to substantial cost impact on the developer and evaluator activities, and also because anything other than the simplest of products is likely to be too complex to submit to current state-of-the-art techniques for formal analysis. (See this introduction to the basic features of the Common Criteria). 5. Despite Professor Spafford's complaints about the intrusion of mere cost considerations into software purchase decisions, in the real-world, resources are limited and trade-offs are inescapable. Developers will limit functionality in an effort to limit the costs of certification or just to make certification practical. If a limited certified operating system causes the complexity of applications to increase and the reliability of those applications to decrease, use of that software may have a negative effect on the security of the whole system. Is it more or less dangerous to use Linux to control a power plant than it is to use an EAL7 (say) OS? Suppose the EAL7 OS comes with no device drivers, costs enough to reduce the amount of test time that can be used in development or the number of trained operators used to monitor the plant, and requires application developers to produce their own math library. Suppose the Linux system is not connected to the network. Suppose the EAL7 evaluation is against a specification that does not cover the most likely threats. The answer is: you better do a real whole systems security analysis instead of relying on buzzwords. 6. Finally, I want to point out that the mere existence of a "certified" version of some company's operating system does not mean anything about the other software produced by that company. For both Common Criteria and the FAA's DO-178 reliability certification, the general practice is to set up a separate development team and a separate, limited, product line. The Common Criteria documents even identify EAL4 as the highest level of assurance possible on a "retrofit" to an existing system - making it very unlikely that a EAL7 product is in the main product line. Even the military projects that GreenHills CEO Dan O'Dowd cites often are not able to bear the costs and/or the limitations of the certified product lines, and so purchase the standard commercial versions which receive PR benefits, but not necessarily any reliability or security benefits, from the certified product lines. The interesting comparison between Linux or some other solution is to the actual products being used, not the highest assurance component sold by the vendor. Software security requires strong engineering and solid cost/benefit analysis, even though that is probably not the best marketing tactic, and it means we have to admit that there are no magic bullets to make the problem go away. Copyright © 2004 Victor Yodaiken, FSMLabs. Reproduced by LinuxDevices.com with permission. Related Stories:
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