When running on an ARM platform Xen was not correctly configuring the
hardware virtualisation platform and therefore did not prevent guests
from accessing various hardware features including cache control,
coprocessors, debug registers and various processor specific
registers.

By accessing these hardware facilities a malicious or buggy guest may
be able to cause various issues, including crashing the host, crashing
other guests (including control domains) and data corruption.

Privilege escalation is not thought to be possible but has not been
ruled out.

References:
http://www.openwall.com/lists/oss-security/2014/04/22/9