The privilege check applied to hypercall attempts by a HVM guest only refused access from ring 3; rings 1 and 2 were allowed through.
Code running in the intermediate privilege rings of HVM guest OSes may be able to elevate its privileges inside the guest by careful hypercall use.
As far as we are aware no mainstream OS (Linux, Windows, BSD) make use of these rings.
Red Hat would like to thank the Xen project for reporting this issue. Upstream acknowledges Jan Beulich as the original reporter.
This issue did affect the versions of the kernel-xen package as shipped with Red Hat Enterprise Linux 5, however please note that none of the guest operating system supported in Red Hat Enterprise Linux 5 are using the vulnerable set of rings of protection for privilege separation.
This issue did not affect the versions of the Linux kernel as shipped with Red Hat Enterprise Linux 6 and Red Hat Enterprise MRG as we did not have support for Xen hypervisor.
This is now public:
Created xen tracking bugs for this issue:
Affects: fedora-all [bug 1034923]
xen-4.2.3-10.fc19 has been pushed to the Fedora 19 stable repository. If problems still persist, please make note of it in this bug report.
xen-4.2.3-10.fc18 has been pushed to the Fedora 18 stable repository. If problems still persist, please make note of it in this bug report.
xen-4.3.1-6.fc20 has been pushed to the Fedora 20 stable repository. If problems still persist, please make note of it in this bug report.
This issue has been addressed in following products:
Red Hat Enterprise Linux 5
Via RHSA-2014:0285 https://rhn.redhat.com/errata/RHSA-2014-0285.html