CVE-2020-25601

An issue was discovered in Xen through 4.14.x. There is a lack of
preemption in evtchn_reset() / evtchn_destroy(). 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. Malicious or buggy guest kernels can mount
a Denial of Service (DoS) attack affecting the entire system. All Xen
versions are vulnerable in principle. Whether versions 4.3 and older are
vulnerable depends on underlying hardware characteristics.

Notes