Guest effectable page reference leak in MMU_MACHPHYS_UPDATE handling

ISSUE DESCRIPTION

An error handling path in the processing of MMU_MACHPHYS_UPDATE failed to drop a page reference which was acquired in an earlier processing step.

IMPACT

Malicious or buggy stub domain kernels or tool stacks otherwise living outside of Domain0 can mount a denial of service attack which, if successful, can affect the whole system.
Only domains controlling HVM guests can exploit this vulnerability. (This includes domains providing hardware emulation services to HVM guests.)

VULNERABLE SYSTEMS

Xen versions from at least 3.2.x onwards are vulnerable on x86 systems. Older versions have not been inspected. ARM systems are not vulnerable.
This vulnerability is only applicable to Xen systems using stub domains or other forms of disaggregation of control domains for HVM guests.