Control-Flow Restrictor: Compiler-based CFI for iOS

Jannik Pewny, Thorsten Holz

Annual Computer Security Applications Conference (ACSAC), New Orleans, USA, December 2013


Abstract

Runtime attacks that exploit software vulnerabilities are still an important concern nowadays. Even smartphone operating systems such as Apple's iOS are affected by such attacks since the system is implemented in Objective-C, a programming language that enables attacks such as buffer overflows. As a generic protection technique against a whole class of attacks, control-flow integrity (CFI) offers some interesting properties. Recent work demonstrated that CFI can be implemented on iOS by patching the binary during the loading process and adding an instrumentation layer that enforces CFI during runtime. However, this approach is of little practical value since it requires a jailbroken device, which hinders wide employment. Furthermore, binary patching has a certain performance impact.

In this paper, we show how CFI can be implemented directly within a compiler, making the approach widely deployable on all kinds of iOS devices. We extend the LLVM compiler and add our CFI enforcement approach during the compilation phase of a given app. An empirical evaluation shows that the size and performance overhead is reasonable.

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Tags: Compiler, Control Flow Integrity, Mobile Security