When Top-down Meets Bottom-up: Detecting and Exploiting Use-After-Cleanup Bugs in Linux Kernel

When a device is detached from the system, Use-After-Cleanup (UAC) bugs can occur because a running kernel thread may be unaware of the device detachment and attempt to use an object that has been released by the cleanup thread. Our investigation suggests that an attacker can exploit the UAC bugs to obtain the capability of arbitrary code execution and privilege escalation, which receives little attention from the community. While existing tools mainly focus on well-known concurrency bugs like data race, few target UAC bugs.In this paper, we propose a tool named UACatcher to systematically detect UAC bugs. UACatcher consists of three main phases. It first scans the entire kernel to find target layers. Next, it adopts the context- and flow-sensitive inter-procedural analysis and the points-to analysis to locate possible free (deallocation) sites in the bottom-up cleanup thread and use (dereference) sites in the top-down kernel thread that can cause UAC bugs. Then, UACatcher uses the routine switch point algorithm which counts on the synchronizations and path constraints to detect UAC bugs among these sites and estimate exploitable ones. For exploitable bugs, we leverage the pseudoterminal-based device emulation technique to develop practical exploits.We have implemented a prototype of UACatcher and evaluated it on 5.11 Linux kernel. As a result, our tool successfully detected 346 UAC bugs, which were reported to the community (277 have been confirmed and fixed and 15 CVEs have been assigned). Additionally, 13 bugs are exploitable, which can be used to develop working exploits that gain the arbitrary code execution primitive in kernel space and achieve the privilege escalation. Finally, we discuss UACatcher’s limitations and propose possible solutions to fix and prevent UAC bugs.