lack of preemption in evtchn_reset() / evtchn_destroy()

ISSUE DESCRIPTION

In particular the FIFO event channel model allows guests to have a large number of event channels active at a time. Closing all of these when resetting all event channels or when cleaning up after the guest may take extended periods of time. So far there was no arrangement for preemption at suitable intervals, allowing a CPU to spend an almost unbounded amount of time in the processing of these operations.

IMPACT

Malicious or buggy guest kernels can mount a Denial of Service (DoS) attack affecting the entire system.

VULNERABLE SYSTEMS

All Xen versions are vulnerable in principle. Whether versions 4.3 and older are vulnerable depends on underlying hardware characteristics.