xen-kernel -- x86: Missing SMAP whitelisting in 32-bit exception / event delivery

The Xen Project reports:

Supervisor Mode Access Prevention is a hardware feature designed
to make an Operating System more robust, by raising a pagefault
rather than accidentally following a pointer into userspace.
However, legitimate accesses into userspace require whitelisting,
and the exception delivery mechanism for 32bit PV guests wasn’t
whitelisted.
A malicious 32-bit PV guest kernel can trigger a safety check,
crashing the hypervisor and causing a denial of service to other
VMs on the host.