Статическая верификация ошибок использования памяти в модулях ядра ОС Linux

Abstract

Memory errors in Linux kernel drivers are a kind of serious bugs that can lead to dangerous consequences but such errors are hard to detect. This article describes static verification that aims at finding all errors under certain assumptions. Static verification of industrial projects such as the Linux kernel requires additional effort. Limitations of current tools for static verification disallow to analyze the Linux kernel as a whole, so we use a simplified automatically generated environment model. This model introduces inaccuracy, but provides ability for verification. In addition, we allow absent definitions for some functions which results in incomplete ANSI C programs. The current work proposes an approach to reveal issues with memory usage in such incomplete programs. Our static verification technique is based on Symbolic Memory Graphs (SMG) with extensions aiming to reduce a false alarm rate. We introduced an on-demand memory conception for simplification of kernel API models and implemented this conception in static verification tool CPAchecker. Also, we changed precision of a CPAchecker memory model from bytes to bits and supported structure alignment similar to the GCC compiler. We implemented the predicate extension for SMG to improve accuracy of the analysis. We verified of Linux kernel 4.11.6 and 4.16.10 with help of the Klever verification framework with CPAchecker as a verification engine. Manual analysis of warnings produced by Klever revealed 78 real bugs in drivers. We have made patches to fix 33 of them.