Many smartphones now deploy conventional operating systems, so the rootkit
attacks so prevalent on desktop and server systems are now a threat to
smartphones. While researchers have advocated using virtualization to detect
and prevent attacks on operating systems (e.g., VM introspection and trusted
virtual domains), virtualization is not practical on smartphone systems due to
the lack of virtualization support and/or the expense of virtualization.
Current smartphone processors do have hardware support for running a protected
environment, such as the ARM TrustZone extensions, but such hardware does not
control the operating system operations sufficiently to enable VM
introspection. In particular, a conventional operating system running with
TrustZone still retains full control of memory management, which a rootkit can
use to prevent traps on sensitive instructions or memory accesses necessary for
effective introspection. In this paper, we present SPROBES, a novel primitive
that enables introspection of operating systems running on ARM TrustZone
hardware. Using SPROBES, an introspection mechanism protected by TrustZone can
instrument individual operating system instructions of its choice, receiving an
unforgeable trap whenever any SPROBE is executed. The key challenge in
designing SPROBES is preventing the rootkit from removing them, but we identify
a set of five invariants whose enforcement is sufficient to restrict rootkits
to execute only approved, SPROBE-injected kernel code. We implemented a
proof-of-concept version of SPROBES for the ARM Fast Models emulator,
demonstrating that in Linux kernel 2.6.38, only 12 SPROBES are sufficient to
enforce all five of these invariants. With SPROBES we show that it is possible
to leverage the limited TrustZone extensions to limit conventional kernel
execution to approved code comprehensively.Comment: In Proceedings of the Third Workshop on Mobile Security Technologies
(MoST) 2014 (http://arxiv.org/abs/1410.6674