Buddy Stacks: Protecting Return Addresses with Efficient Thread-Local Storage and Runtime Re-Randomization

Abstract

Shadow stacks play an important role in protecting return addresses to mitigate ROP attacks. Parallel shadow stacks, which shadow the call stack of each thread at the same constant offset for all threads, are known not to support multi-threading well. On the other hand, compact shadow stacks must maintain a separate shadow stack pointer in thread-local storage (TLS), which can be implemented in terms of a register or the per-thread Thread-Control-Block (TCB), suffering from poor compatibility in the former or high performance overhead in the latter. In addition, shadow stacks are vulnerable to information disclosure attacks. In this paper, we propose to mitigate ROP attacks for single- and multi-threaded server programs running on general-purpose computing systems by using a novel stack layout, called a buddy stack (referred to as Bustk), that is highly performant, compatible with existing code, and provides meaningful security. These goals are met due to three novel design aspects in Bustk. First, Bustk places a parallel shadow stack just below a thread’s call stack (as each other’s buddies allocated together), avoiding the need to maintain a separate shadow stack pointer and making it now well-suited for multi-threading. Second, Bustk uses an efficient stack-based thread-local storage mechanism, denoted STK-TLS, to store thread-specific metadata in two TLS sections just below the shadow stack in dual redundancy (as each other’s buddies), so that both can be accessed and updated in a lightweight manner from the call stack pointer rsp alone. Finally, Bustk re-randomizes continuously (on the order of milliseconds) the return addresses on the shadow stack by using a new microsecond-level runtime re-randomization technique, denoted STK-MSR. This mechanism aims to obsolete leaked information, making it extremely unlikely for the attacker to hijack return addresses, particularly against a server program that sits often tens of milliseconds away from the attacker. Our evaluation using web servers, Nginx and Apache Httpd, shows that Bustk works well in terms of performance, compatibility, and security provided, with its parallel shadow stacks incurring acceptable memory overhead for real-world applications and its STK-TLS mechanism costing only two pages per thread. In particular, Bustk can protect the Nginx and Apache servers with an adaptive 1-ms re-randomization policy (without observable overheads when IO is intensive, with about 17,000 requests per second). In addition, we have also evaluated Bustk using other non-server applications, Firefox, Python, LLVM, JDK and SPEC CPU2006, to demonstrate further the same degree of performance and compatibility provided, but the protection provided for, say, browsers, is weaker (since network-access delays can no longer be assumed).