No one would accept an engineering approach for a suspension bridge that was based on guessing at steel cable sizes and then loading the deck to see whether it collapsed, or sizing elevator motors by trying them out to see whether they caught fire. And yet these approaches are exactly analogous to how software developers often approach their work.
The development cycle for software is largely reactive, where code is developed using an informal agile approach, with no risk mitigation and no coding guidelines. Executables are then subjected to performance, penetration, load and functional tests to try to find the vulnerabilities that almost certainly result. The hope is that all issues will be found and holes adequately plugged. But whether a product is safety-critical or not, it’s time for software developers to embrace the same sound processes as other engineering disciplines. That consists of defining requirements, creating a design to fulfil those requirements, developing a product that is true to the design, and then testing it to show that it is. Even in this process, developers have alternatives to choose from, such as CERT C’s application-centric approach to the detection of issues, versus MISRA’s ethos of using design patterns to prevent their introduction. With either approach, software developers must learn to design in security up front rather than hope to remove insecurity later.
Safe & Secure Application Code Development
The traditional approach to secure software development is mostly a reactive one – develop the software and then use penetration, fuzz and functional test to expose any weaknesses. In isolation, however, that is not good enough to comply with a functional safety standard such as DO-178C (in the aerospace sector), IEC 62304 (medical devices) or ISO 26262 (automotive). These demand that security factors with a safety implication are considered from the outset, because a safety-critical system cannot be safe if is not secure.
Using ISO 26262 as an example, Figure 1 illustrates a V-model with cross-references to the ISO 26262 standard and tools likely to be deployed at each phase in the development of complex automotive software. (Other process models such as agile and waterfall can be equally well-supported.) The application of such a process does not negate the value of penetration and fuzz testing, but allows these techniques to provide evidence of system robustness rather than exposing their vulnerabilities.
The outputs from the system design phase (top left) includes technical safety requirements refined and allocated to hardware and software. In a connected system, these include security requirements because the action to be taken to deal with each safety-threatening security issue needs to be proportionate to the risk. Maintaining traceability between these requirements and the products of subsequent phases, however, can cause a major project management headache.
The specification of software requirements involves their derivation from the system design, isolating the software-specific elements and detailing the evolution process of lower-level, software-related requirements, including those with a security-related element.
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Figure 2: Graphical representation of Control and Data Flow as depicted in the LDRA tool suite (Source: LDRA)
Next comes the software architectural design phase , perhaps using a UML graphical representation. Static analysis tools provide graphical representations of the relationship between code components for comparison with the intended design (Figure 2).
Figure 3 illustrates a typical example of a table from ISO 26262-6:2011 relating to software design and implementation . It shows the coding and modelling guidelines to be enforced during implementation, along with an indication of where compliance can be confirmed with the automated tools.
The “use of language subset” (topic 1b) exemplifies the impact of security considerations. Language subsets have traditionally been viewed as an aid to safety, but security enhancements to the MISRA C:2012 standard and security-specific standards such as CWE and CERT C reflect an increasing interest in the role they have to play in combating security issues. These can also be checked by means of static analysis (Figure 4).
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Figure 4: Coding standards violations as represented by the LDRA tool suite (Source: LDRA)
Dynamic analysis techniques (involving the execution of some or all of the code) are applicable to unit, integration and system testing . Unit testing focuses on particular software procedures or functions in isolation, whereas integration testing ensures that safety, security and functional requirements are met when units are working together in accordance with the software architectural design.
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Figure 5: Unit testing with the LDRA tool suite (Source: LDRA)
Figure 5 shows how the software interface is exposed at the function scope, allowing the developer to enter inputs and expected outputs to form f a test harness. That harness is then compiled and executed on the target hardware, and actual and expected outputs compared. This technique shows functional correctness in accordance with requirements, as well as resilience to issues such as border conditions, null pointers and default switch cases – all important security considerations.
In addition to showing that software functions correctly, dynamic analysis is used to generate structural coverage metrics. Both MISRA C:2012 (Dir 3.1) and the security standard CWE (Figure 6) require that code coverage analysis is used to ensure that there is no hidden functionality that can increase an application’s attack surface and expose weaknesses.
Choosing a Language Subset
Although there are several language subsets (or less formally, “coding standards”) to choose from, these have traditionally been focused primarily on safety rather than security. With the advent of the Industrial Internet of Things and connected cars and medical devices, that focus has shifted towards security since these systems, once naturally secure through isolation, are now increasingly accessible to aggressors.
There are, however, subtle differences between the differing subsets.
MISRA C:2012 states that “MISRA C should be adopted from the outset of a project. If a project is building on existing code that has a proven track record then the benefits of compliance with MISRA C may be outweighed by the risks of introducing a defect when making the code compliant.” This contrasts with the assertion of CERT C that although “the priority of this standard is to support new code development…. A close-second priority is supporting remediation of old code.”
The level of risk involved with the compromise of the system impacts the approach. While the retrospective application of any subset is better than nothing, it does not represent best practice.
Relevance to safety, high-integrity and high-reliability systems
MISRA C:2012 “define[s] a subset of the C language in which the opportunity to make mistakes is either removed or reduced. Many standards for the development of safety-related software require or recommend a language subset, and this can also be used to develop any application with high-integrity or high-reliability requirements.” The accurate implication of that statement is that MISRA C was always appropriate for security-critical applications even before the security enhancements introduced by MISRA C:2012 Amendment 1.
CERT C attempts to be more all-encompassing, covering application programming (e.g., POSIX) as well as the C language. That is reflected in its introductory suggestion that “safety-critical systems typically have stricter requirements than are imposed by this standard … However, the application of this coding standard will result in high-quality systems that are reliable, robust, and resistant to attack.”
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The primary purpose of a requirements-driven software-development process as exemplified by ISO 26262 is to control the development process to minimize the possibility of error or inconsistency. Although that is theoretically possible by manual means, it is far more effective if software tools are used to automate the process.
In the case of static analysis tools, that requires that the rules can be checked algorithmically. Compare, for example, the excerpts shown in Figure 7a and 7b, both of which address the same issue. The approach taken by MISRA (Figure 7a) is to prevent the issue by disallowing the inclusion of the pertinent construct. CERT C (Figure 7b) instead asserts that the developer should “be aware” of it.
The CERT C approach is more flexible; something of particular value if rules are applied retrospectively. MISRA C:2012 is more draconian, yet by avoiding the side effects altogether the resulting code is certain to be more portable. More importantly, it can be automatically checked by a static analysis tool. It is simply not possible for a tool to check whether a developer is “aware” of side effects – and less possible still to ascertain whether “awareness” equals “understanding.”
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Figure 7a: MISRA C:2012 excerpt demonstrating the contrast in approaches concerning the decidability of coding rules (Source: MISRA Guidelines for the Use of the C Language in Critical Systems, ISBN 978-1-906400-10-1 (paperback), ISBN 978-1-906400-11-8 (PDF), March 2013)
Precision of rule definitions
The more precisely defined approach of MISRA also addresses the issue of language misunderstanding more convincingly than CERT C. Evidence suggests that there are characteristics of the C language responsible for most of the defects found in C source code, such that around 80% of software defects are caused by the incorrect usage of about 20% of the available C or C++ language constructs. By restricting the use of the language to avoid problematic parts, developers can avoid writing defects into the code and increase software quality.
Figure 8 uses the handling of variadic functions to illustrate how the MISRA approach (Figure 8a) differs from that of CERT C (Figure 8b). CERT C calls for developers to “understand” the associated type issues, but doesn’t suggest how a situation might be handled where a developer harbors a misunderstanding.
A counter-argument might be that there are developers who are aware of the type issues associated with variadic functions, who make good use of them, and who may feel restricted in the prohibition of their use. However, for highly safety- or security-critical systems, MISRA asserts that because the “opportunity to make mistakes is either removed or reduced,” that is a price worth paying.
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Figure 8a: MISRA C:2012 excerpt c omparing differing precision of rule definition (Source: MISRA Guidelines for the Use of the C Language in Critical Systems, ISBN 978-1-906400-10-1 (paperback), ISBN 978-1-906400-11-8 (PDF), March 2013)
A question of priorities
The correct application of either CERT C or MISRA C:2012 will certainly result in more secure code than if neither were applied. However, for safety- or security-critical applications, MISRA C is less error-prone because it is designed for such systems, with stricter, more decidable rules. CERT C is more tolerant and might be a pragmatic choice for applications that are not critical but will be connected to the internet for the first time.
Final Thoughts: Bi-Directional Traceability
The principle of bi-directional traceability runs throughout the V-models referenced in DO-178C, IEC 62304 and ISO 26262, with each development phase required to accurately reflect the one before it. In theory, if the exact sequence of the standard is adhered to, then the requirements will never change and tests will never throw up a problem. But life’s not like that.
For example, it is easy to imagine these processes as they relate to a green-field project. But what if there is a need to integrate many different subsystems? What if some of those are pre-existing, with requirements defined in widely different formats? What if some of those systems were written with no security in mind, assuming an isolated system? And what if different subsystems are in different development phases?
Then there is the issue of requirements changes. What if the client has a change of heart? A bright idea? Advice from a lawyer that existing approaches could be problematic?
Should changes become necessary, revised code would need to be reanalysed statically, and all impacted unit and integration tests would need to be re-run (regression tested). Although that can result in a project management nightmare at the time, in an isolated application it lasts little longer than the time the product is under development.
Connectivity, with its inherent need for security, changes all that. Whenever a new vulnerability is discovered, there is the potential for a resulting change of requirement to cater for it, coupled with the additional pressure of knowing that a speedy response could be critically important if products are not to be compromised in the field. Indeed, many IoT systems are difficult to patch once in service.
Automated bi-directional traceability links requirements from different sources through to design, code and test. The impact of any requirements changes – or failed test cases – can be assessed by impact analysis and addressed accordingly. Artefacts can be automatically re-generated to present evidence of continued compliance to the appropriate standard.
During the development of a traditional, isolated system, that is useful enough. But connectivity demands the ability to respond to vulnerabilities because each newly discovered vulnerability implies a changed or new requirement, and one to which an immediate response is needed – even though the system itself may not have been touched by development engineers for some time. Being able to isolate what is needed and automatically test only the functions implemented becomes much more significant.
Mark Pitchford has over 25 years’ experience in software development for engineering applications. He has worked on many significant industrial and commercial projects in development and management, both in the UK and internationally. Since 2001, he has worked with development teams looking to achieve compliant software development in safety and security critical environments, working with standards such as DO-178, IEC 61508, ISO 26262, IIRA and RAMI 4.0. Mark earned his Bachelor of Science degree at Trent University, Nottingham, and he has been a Chartered Engineer for over 20 years. He now works as Technical Specialist with LDRA Software Technology.