Bug 1304628 (CVE-2016-2271, xsa170)

ISSUE DESCRIPTION
=================

VMX refuses attempts to enter a guest with an instruction pointer which doesn't satisfy certain requirements. In particular, the instruction pointer needs to be canonical when entering a guest currently in 64-bit mode. This is the case even if the VM entry information specifies an exception to be injected immediately (in which case the bad instruction pointer would possibly never get used for other than pushing onto the exception handler's stack). Provided the guest OS allows user mode to map the virtual memory space immediately below the canonical/non- canonical address boundary, a non-canonical instruction pointer can result even from normal user mode execution. VM entry failure, however, is fatal to the guest.

IMPACT
======

Malicious HVM guest user mode code may be able to crash the guest.

VULNERABLE SYSTEMS
==================

Only systems using Intel or Cyrix CPUs are affected. ARM and AMD systems are unaffected. Only HVM guests are affected.

MITIGATION
==========

Running only PV guests will avoid this vulnerability. Running HVM guests on only AMD hardware will also avoid this
vulnerability.


External References:

http://xenbits.xen.org/xsa/advisory-170.html

Acknowledgements:

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