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.
HVM/PVH guest userspace code may be able to crash the guest, resulting
in a guest Denial of Service.
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
Running only PV guests will avoid this vulnerability.
Running HVM guests on only AMD hardware will also avoid this
This issue was discovered by Håkon Alstadheim and diagnosed as a
security issue by Andrew Cooper of Citrix.
DEPLOYMENT DURING EMBARGO
Deployment of the patches and/or mitigations described above (or
others which are substantially similar) is permitted during the
embargo, even on public-facing systems with untrusted guest users and
But: Distribution of updated software is prohibited (except to other
members of the predisclosure list).
Predisclosure list members who wish to deploy significantly different
patches and/or mitigations, please contact the Xen Project Security
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