The Virtual Performance Measurement Unit feature has been documented as unsupported on Intel CPUs. Further issues have been found or are suspected which would also (or exclusively) affect AMD CPUs. Use of the feature may have unknown effects, ranging from information leaks through Denial of Service to privilege escalation.
Only systems which enable the VPMU feature are affected. That is, only systems with a `vpmu' setting on the hypervisor command line. Xen versions from 3.3 onwards are affected. Only x86 systems are affected. ARM systems do not currently implement vPMU and are therefore currently unaffected. In Xen versions prior to 4.6 only HVM guests can take advantage of this unsupported functionality. In Xen versions from 4.6 onwards all guest kinds can use this unsupported functionality. Not enabling vPMU support (by omitting the "vpmu" hypervisor command line option) will avoid using and exposing the unsupported functionality.
Created xen tracking bugs for this issue:
Affects: fedora-all [bug 1285351]
Red Hat would like to thank the Xen project for reporting this issue.
xen-4.5.2-5.fc23 has been pushed to the Fedora 23 stable repository. If problems still persist, please make note of it in this bug report.
xen-4.5.2-5.fc22 has been pushed to the Fedora 22 stable repository. If problems still persist, please make note of it in this bug report.
This issue does not affect the Linux kernel packages as shipped with Red Hat
Enterprise Linux 5,6,7, MRG-2 and realtime.
The code that enables VPMU as a valid instructions are not included in the
initial version of xen that ships with Red Hat Enterprise Linux 5.
At this time this flaw only affects systems with the hypervisor booted with the vpmu parameter, which is not supported by the version shipping with Red Hat Enterprise Linux 5.