Citrix XEN Vulnerability - version 4.1 and upwards
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.
Find Xen Security Advisory and related fix info here - http://xenbits.xen.org/xsa/advisory-108.html