Several long latency operations are not preemptible

ISSUE DESCRIPTION

Page table manipulation operations for PV guests can take significant amounts of time, as they require all present branches to have their type (and thus contents) verified. While the most frequently used operations had been made preemptible in the past, some code paths involving potentially deep page table traversal were still trying to do their entire work in a single step.

IMPACT

Malicious or buggy PV guest kernels can mount a denial of service attack affecting the whole system.

VULNERABLE SYSTEMS

All Xen versions are vulnerable.
The vulnerability is only exposed by PV guests.