A Reliable and Practical Approach to Kernel Attack Surface Reduction of Commodity OS
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Commodity OS kernels are known to have broad attack surfaces due to the large code base and the numerous features such as device drivers. For a real-world use case (e.g., an Apache Server), many kernel services are unused and only a small amount of kernel code is used. Within the used code, a certain part is invoked only at the runtime phase while the rest are executed at startup and/or shutdown phases in the kernel's lifetime run. In this paper, we propose a reliable and practical system, named KASR, which transparently reduces attack surfaces of commodity OS kernels at the runtime phase without requiring their source code. The KASR system, residing in a trusted hypervisor, achieves the attack surface reduction through two reliable steps: (1) deprives unused code of executable permissions, and (2) transparently segments used code and selectively activates them according to their phases. KASR also prevents unknown code from executing in the kernel space, and thus it is able to defend against all kernel code injection attacks. We implement a prototype of KASR on the Xen-4.8.2 hypervisor and evaluate its security effectiveness on Linux kernel-4.4.0-87-generic. Our evaluation shows that KASR reduces kernel attack surface by 64 vulnerabilities and 66 drivers (including their related CVEs) from executing. Besides, KASR successfully detects and blocks all 6 real-world kernel rootkits. We measure its performance overhead with three benchmark tools (i.e., SPECINT, httperf and bonnie++). The experimental results indicate that KASR imposes less than 1 averaged performance overhead compared to an unmodified Xen hypervisor.
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