p2m lock starvation

ISSUE DESCRIPTION

The current read/write lock implementation is read-biased, which allows a consistent stream of readers to starve writers indefinitely. There are certain rwlocks where guests are capable of applying arbitrary read pressure.

IMPACT

A malicious guest administrator can deny service to other tasks. If the NMI watchdog is active, a timeout might be triggered, resulting in a host crash.

VULNERABLE SYSTEMS

Xen 4.2 and later systems are vulnerable.
Xen 4.1 and earlier are not vulnerable in normal configurations. 4.1 and earlier are vulnerable only insofar as features are used which have already been explicitly discounted for security support purposes (TMEM, see XSA-15; XSM-based radical disaggregation, see XSA-77).
Only x86 systems offer avenues for attacking this vulnerability. ARM systems do not and are therefore not vulnerable.