Hypercalls exposed to privilege rings 1 and 2 of HVM guests

ISSUE DESCRIPTION

The privilege check applied to hypercall attempts by a HVM guest only refused access from ring 3; rings 1 and 2 were allowed through.

IMPACT

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.

VULNERABLE SYSTEMS

Xen 3.0.3 and later are vulnerable. Xen 3.0.2 and earlier are not vulnerable.