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.

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.)

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.


Acknowledgements:

Red Hat would like to thank the Xen project for reporting this issue.


External References:

http://xenbits.xen.org/xsa/advisory-113.html