PrivWatcher: Non-bypassable Monitoring and Protection of Process Credentials from Memory Corruption Attacks

Abstract

Commodity operating systems kernels are typically implemented using low-level unsafe languages, which leads to the inevitability of memory corruption vulnerabilities. Multiple defense techniques are widely adopted to mitigate the impact of memory corruption on executable code and control data. Nevertheless, there has not been much attention to defend against corruption of non-control data despite the fact that previous incidents of kernel exploitation showed that corrupting non-control data is a real threat. We present PrivWatcher, a framework for monitoring and protecting the integrity of process credentials and their usage contexts from memory corruption attacks. PrivWatcher solves multiple challenges to achieve this objective. It introduces techniques to isolate and protect the data that define process credentials and guarantee the locality of this data within the protected memory. Then, by adopting a dual reference monitor model, it guarantees the Time of Check To Time of Use (TOCTTOU) consistency between verification and usage contexts for process credentials. Moreover, it provides a secure mechanism that allows the presumably protected kernel code to verify the protected data without relying on unprotected data fields. PrivWatcher provides non-bypassable integrity assurances for process credentials and can be adapted to enforce a variety of integrity policies. In this paper, we demonstrate an application of PrivWatcher that enforces the original semantics of the OS kernel's access control policy: a change in process privileges is legitimate only if an uncompromised kernel would have allowed it. We implemented a PrivWatcher prototype to protect Ubuntu Linux running on x86-64. Evaluation of our prototype showed that PrivWatcher is effective and efficient.