VMX: VMentry failure with debug exceptions and blocked states

ISSUE DESCRIPTION

Please see XSA-260 for background on the MovSS shadow: <a href=“http://xenbits.xen.org/xsa/advisory-260.html”>http://xenbits.xen.org/xsa/advisory-260.html</a>
Please see XSA-156 for background on the need for #DB interception: <a href=“http://xenbits.xen.org/xsa/advisory-156.html”>http://xenbits.xen.org/xsa/advisory-156.html</a>
The VMX VMEntry checks does not like the exact combination of state which occurs when #DB in intercepted, Single Stepping is active, and blocked by STI/MovSS is active, despite this being a legitimate state to be in. The resulting VMEntry failure is fatal to the guest.

IMPACT

HVM/PVH guest userspace code may be able to crash the guest, resulting in a guest Denial of Service.

VULNERABLE SYSTEMS

All versions of Xen are affected.
Only systems supporting VMX hardware virtual extensions (Intel, Cyrix or Zhaoxin CPUs) are affected. Arm and AMD systems are unaffected.
Only HVM/PVH guests are affected. PV guests cannot leverage the vulnerability.