  Attacks on the microarchitecture of modern processors have become a practical
threat to security and privacy in desktop and cloud computing. Recently, cache
attacks have successfully been demonstrated on ARM based mobile devices,
suggesting they are as vulnerable as their desktop or server counterparts. In
this work, we show that previous literature might have left an overly
pessimistic conclusion of ARM's security as we unveil AutoLock: an internal
performance enhancement found in inclusive cache levels of ARM processors that
adversely affects Evict+Time, Prime+Probe, and Evict+Reload attacks. AutoLock's
presence on system-on-chips (SoCs) is not publicly documented, yet knowing that
it is implemented is vital to correctly assess the risk of cache attacks. We
therefore provide a detailed description of the feature and propose three ways
to detect its presence on actual SoCs. We illustrate how AutoLock impedes
cross-core cache evictions, but show that its effect can also be compensated in
a practical attack. Our findings highlight the intricacies of cache attacks on
ARM and suggest that a fair and comprehensive vulnerability assessment requires
an in-depth understanding of ARM's cache architectures and rigorous testing
across a broad range of ARM based devices.
