xen-kernel -- x86 shadow pagetables: address width overflow

The Xen Project reports:

In the x86 shadow pagetable code, the guest frame number of a
superpage mapping is stored in a 32-bit field. If a shadowed guest
can cause a superpage mapping of a guest-physical address at or
above 2^44 to be shadowed, the top bits of the address will be lost,
causing an assertion failure or NULL dereference later on, in code
that removes the shadow.
A HVM guest using shadow pagetables can cause the host to crash.

A PV guest using shadow pagetables (i.e. being migrated) with PV
superpages enabled (which is not the default) can crash the host, or
corrupt hypervisor memory, and so a privilege escalation cannot be
ruled out.