[XSA-391] Rogue backends can cause DoS of guests via high frequency events

ISSUE DESCRIPTION

Xen offers the ability to run PV backends in regular unprivileged
guests, typically referred to as "driver domains". Running PV backends
in driver domains has one primary security advantage: if a driver domain
gets compromised, it doesn't have the privileges to take over the
system.

However, a malicious driver domain could try to attack other guests via
sending events at a high frequency leading to a Denial of Service in the
guest due to trying to service interrupts for elongated amounts of time.

There are three affected backends:
* blkfront patch 1, CVE-2021-28711
* netfront patch 2, CVE-2021-28712
* hvc_xen (console) patch 3, CVE-2021-28713

IMPACT

Potentially malicious PV backends can cause guest DoS due to unhardened
frontends in the guests, even though this ought to have been prevented by
containing them within a driver domain.

VULNERABLE SYSTEMS

All guests being serviced by potentially malicious backends are vulnerable,
even if those backends are running in a less privileged environment. The
vulnerability is not affecting the host, but the guests.