It was found that PV superpage functionality lacks certain validity checks on data being passed to the hypervisor by guests. This is the case for the page identifier (MFN) passed to MMUEXT_MARK_SUPER and MMUEXT_UNMARK_SUPER sub-ops of the HYPERVISOR_mmuext_op hypercall as well as for various forms of page table updates.
Use of the feature, which is disabled by default, may have unknown effects, ranging from information leaks through Denial of Service to privilege escalation.
Only systems which enable the PV superpage feature are affected. That is, only systems with an `allowsuperpage' setting on the hypervisor command line. Note that in Xen 4.0.x and 3.4.x the option is named `allowhugepage'. Xen versions 3.4.0, 3.4.1, and from 4.1 onwards are affected. Only x86 systems are affected. Only PV guests can exploit the vulnerability.
Running only HVM guests will avoid this issue. Not enabling PV superpage support (by omitting the `allowsuperpage' or `allowhugepage' hypervisor command line options) will avoid exposing the issue.