C remains central to our computing infrastructure. It is notionally defined by ISO standards, but in reality the properties of C assumed by systems code and those implemented by compilers have diverged, both from the ISO standards and from each other, and none of these are clearly understood. We make two contributions to help improve this error-prone situation. First, we describe an in-depth analysis of the design space for the semantics of pointers and memory in C as it is used in practice. We articulate many specific questions, build a suite of semantic test cases, gather experimental data from multiple implementations, and survey what C experts believe about the de facto standards. We identify questions where there is a consensus (either following ISO or differing) and where there are conflicts. We apply all this to an experimental C implemented above capability hardware. Second, we describe a formal model, Cerberus, for large parts of C. Cerberus is parameterised on its memory model; it is linkable either with a candidate de facto memory object model, under construction, or with an operational C11 concurrency model; it is defined by elaboration to a much simpler Core language for accessibility, and it is executable as a test oracle on small examples. This should provide a solid basis for discussion of what mainstream C is now: what programmers and analysis tools can assume and what compilers aim to implement. Ultimately we hope it will be a step towards clear, consistent, and accepted semantics for the various use-cases of C.

/pdf/into-the-depths-of-c-elaborating-the-de-facto-standards-3ak41gpa22.pdf

Into the depths of C: elaborating the de facto standards

values hide internal details of the memory such as permissions, padding and object representations. They are therefore used in the external interface of the memory model and throughout the operational semantics. Memory trees, abstract values and bits with permissions can be converted into each other. These conversions are used to define operations internal to the memory model. However, none of these conversions are bijective because different information is materialized in these three data types: Abstract values Memory trees Bits with permissions Permissions X X Padding always E X Variants of union X X Mathematical values Xvalues Memory trees Bits with permissions Permissions X X Padding always E X Variants of union X X Mathematical values X This table indicates that abstract values and sequences of bits are complementary. Memory trees are a middle ground, and therefore suitable to describe both the lowlevel and high-level aspects of the C memory.

/pdf/the-c-standard-formalized-in-coq-5kv78rakm6.pdf

The C standard formalized in Coq

Good programming languages provide helpful abstractions for writing secure code, but the security properties of the source language are generally not preserved when compiling a program and linking it with adversarial code in a low-level target language (e.g., a library or a legacy application). Linked target code that is compromised or malicious may, for instance, read and write the compiled program^{\prime}s data and code, jump to arbitrary memory locations, or smash the stack, blatantly violating any source-level abstraction. By contrast, a fully abstract compilation chain protects source-level abstractions all the way down, ensuring that linked adversarial target code cannot observe more about the compiled program than what some linked source code could about the source program. However, while research in this area has so far focused on preserving observational equivalence, as needed for achieving full abstraction, there is a much larger space of security properties one can choose to preserve against linked adversarial code. And the precise class of security properties one chooses crucially impacts not only the supported security goals and the strength of the attacker model, but also the kind of protections a secure compilation chain has to introduce. We are the first to thoroughly explore a large space of formal secure compilation criteria based on robust property preservation, i.e., the preservation of properties satisfied against arbitrary adversarial contexts. We study robustly preserving various classes of trace properties such as safety, of hyperproperties such as noninterference, and of relational hyperproperties such as trace equivalence. This leads to many new secure compilation criteria, some of which are easier to practically achieve and prove than full abstraction, and some of which provide strictly stronger security guarantees. For each of the studied criteria we propose an equivalent "property-free" characterization that clarifies which proof techniques apply. For relational properties and hyperproperties, which relate the behaviors of multiple programs, our formal definitions of the property classes themselves are novel. We order our criteria by their relative strength and show several collapses and separation results. Finally, we adapt existing proof techniques to show that even the strongest of our secure compilation criteria, the robust preservation of all relational hyperproperties, is achievable for a simple translation from a statically typed to a dynamically typed language.

Journey Beyond Full Abstraction: Exploring Robust Property Preservation for Secure Compilation

The ISO C standard does not specify the semantics of many valid programs that use non-portable idioms such as integer-pointer casts. Recent efforts at formal definitions and verified implementation of the C language inherit this feature. By adopting high-level abstract memory models, they validate common optimizations. On the other hand, this prevents reasoning about much low-level code relying on the behavior of common implementations, where formal verification has many applications. We present the first formal memory model that allows many common optimizations and fully supports operations on the representation of pointers. All arithmetic operations are well-defined for pointers that have been cast to integers. Crucially, our model is also simple to understand and program with. All our results are fully formalized in Coq.

/pdf/a-formal-c-memory-model-supporting-integer-pointer-casts-11qsalkhur.pdf

A formal C memory model supporting integer-pointer casts

Transformations over assembly code are common in many compilers. These transformations are also some of the most bug-dense compiler components. Such bugs could be elim- inated by formally verifying the compiler, but state-of-the- art formally verified compilers like CompCert do not sup- port assembly-level program transformations. This paper presents Peek, a framework for expressing, verifying, and running meaning-preserving assembly-level program trans- formations in CompCert. Peek contributes four new com- ponents: a lower level semantics for CompCert x86 syntax, a liveness analysis, a library for expressing and verifying peephole optimizations, and a verified peephole optimiza- tion pass built into CompCert. Each of these is accompanied by a correctness proof in Coq against realistic assumptions about the calling convention and the system memory alloca- tor. Verifying peephole optimizations in Peek requires prov- ing only a set of local properties, which we have proved are sufficient to ensure global transformation correctness. We have proven these local properties for 28 peephole transfor- mations from the literature. We discuss the development of our new assembly semantics, liveness analysis, representa- tion of program transformations, and execution engine; de- scribe the verification challenges of each component; and detail techniques we applied to mitigate the proof burden.

Verified peephole optimizations for CompCert

https://hal-centralesupelec.archives-ouvertes.fr/hal-02288116/document

Isolating malicious code in Android malware in the wild

—With millions of Android malware samples avail- able, researchers have a large amount of data to perform malware detection and classiﬁcation, specially with the help of machine learning. Thus far, visualization tools focus on single samples or one-to-many comparison, but not a many-to-many approach. In order to exploit the quantity of data from various datasets to obtain meaningful information, we propose DaViz, a visualization tool for Android malware datasets. With the aid of multiple chart types and interactive sample ﬁltering, users can explore different application datasets and compare them. This new tool allows to get a better understanding of the datasets at hand, and help to continue research by narrowing the samples to those of interest based on selected characteristics.

DaViz: Visualization for Android Malware Datasets

The perpetual opposition between antiviruses and malware leads both parties to evolve continuously. On the one hand, antiviruses put in place solutions that are more and more sophisticated and propose more complex detection techniques in addition to the classic signature analysis. This sophistication leads antiviruses to leave more traces of their presence on the machine they protect. To remain undetected as long as possible, malware can avoid executing within such environments by hunting down the modifications left by the antiviruses. This paper aims at determining the possibilities for malware to detect the antiviruses and then evaluating the efficiency of these techniques on a panel of antiviruses that are the most used nowadays. We then collect samples showing this kind of behavior and propose to evaluate a countermeasure that creates false artifacts, thus forcing malware to evade.

Evasive Windows Malware: Impact on Antiviruses and Possible Countermeasures

Analyzing Android applications is essential to review proprietary code and to understand malware behaviors. However, Android applications use obfuscation techniques to slow down this process. These obfuscation techniques are increasingly based on native code. In this paper, we propose OATs’inside, a new analysis tool that focuses on high-level behaviors in order to circumvent native obfuscation techniques transparently. The targeted high-level behaviors are object-level behaviors, i.e. actions performed on Java objects (e.g. field accesses, method calls), regardless of whether these actions are performed using Java or native code. Our system uses a hybrid approach based on dynamic monitoring and trace-based symbolic execution to output control flow graphs (CFGs) for each method of the analyzed application. CFGs are composed of Java-like actions enriched with condition expressions and data flows between actions, giving an understandable representation of any code, even those fully native. OATs’inside spares users the need to dive into low-level instructions, which are difficult to reverse engineer. We extensively compare OATs’inside functionalities against state-of-the-art tools to highlight the benefit when observing native operations. Our experiments are conducted on a real smartphone: we discuss the performance impact of OATs’inside and we demonstrate its practical use on applications containing anti-debugging techniques provided by the OWASP foundation. We also evaluate the robustness of OATs’inside using obfuscated unit tests using the Tigress obfuscator.

/pdf/oatsinside-retrieving-object-behaviorsfrom-native-based-77y1si8k.pdf

OATs’inside: Retrieving Object BehaviorsFrom Native-Based Obfuscated Android Applications

Verifying Android applications' source code is essential to ensure users' security. Due to its complex architecture, Android has specific attack surfaces which the community has to investigate in order to discover new vulnerabilities and prevent as much as possible malicious exploitations. Communication mechanisms are one of the Android components that should be carefully checked and analyzed to avoid data leakage or code injections. Android software components can communicate together using serialization processes. Developers need thereby to indicate manually the transient keyword whenever an object field should not be part of the serialization. In particular, field values encoding memory addresses can leave severe vulnerabilities inside applications if they are not explicitly declared transient. In this study, we propose a novel methodology for automatically detecting, at compilation time, all missing transient keywords directly from Android applications' source code. Our method is based on taint analysis and its implementation provides developers with a useful tool which they might use to improve their code bases. Furthermore, we evaluate our method on a cryptography library as well as on the Telegram application for real world validation. Our approach is able to retrieve previously found vulnerabilities, and, in addition, we find non-exploitable flows hidden within Telegram's code base.

/pdf/preventing-serialization-vulnerabilities-through-transient-3px0ffvshc.pdf

Pierre Wilke

Papers

Isolating malicious code in Android malware in the wild

DaViz: Visualization for Android Malware Datasets

Evasive Windows Malware: Impact on Antiviruses and Possible Countermeasures

OATs’inside: Retrieving Object BehaviorsFrom Native-Based Obfuscated Android Applications

Preventing serialization vulnerabilities through transient field detection