The MSR range specified for APIC use in the x2APIC access model spans 256 MSRs. Hypervisor code emulating read and write accesses to these MSRs erroneously covered 1024 MSRs. While the write emulation path is written such that accesses to the extra MSRs would not have any bad effect (they end up being no-ops), the read path would (attempt to) access memory beyond the single page set up for APIC emulation. A buggy or malicious HVM guest can crash the host or read data relating to other guests or the hypervisor itself. Acknowledgements: Red Hat would like to thank the Xen for reporting this issue.
Statement: Not vulnerable. This issue does not affect the versions of the kernel-xen package as shipped with Red Hat Enterprise Linux 5.
This is public now: http://seclists.org/oss-sec/2014/q4/7
Created xen tracking bugs for this issue: Affects: fedora-all [bug 1148465]
xen-4.4.1-6.fc21 has been pushed to the Fedora 21 stable repository. If problems still persist, please make note of it in this bug report.