x86 null segments not always treated as unusable

ISSUE DESCRIPTION

The Xen x86 emulator erroneously failed to consider the unusability of segments when performing memory accesses.
The intended behaviour is as follows: The user data segment (%ds, %es, %fs and %gs) selectors may be NULL in 32-bit to prevent access. In 64-bit, NULL has a special meaning for user segments, and there is no way of preventing access. However, in both 32-bit and 64-bit, a NULL LDT system segment is intended to prevent access.
On Intel hardware, loading a NULL selector zeros the base as well as most attributes, but sets the limit field to its largest possible value. On AMD hardware, loading a NULL selector zeros the attributes, leaving the stale base and limit intact.
Xen may erroneously permit the access using unexpected base/limit values.
Ability to exploit this vulnerability on Intel is easy, but on AMD depends in a complicated way on how the guest kernel manages LDTs.

IMPACT

An unprivileged guest user program may be able to elevate its privilege to that of the guest operating system.

VULNERABLE SYSTEMS

The vulnerability is only exposed to HVM guests.
ARM systems are NOT vulnerable.
All versions of Xen are affected.
However, we believe that the vulnerability cannot be exploited on Xen 4.7 by completely unprivileged guest processes, unless the VM has been explicitly configured with a non-default cpu vendor string (in xm/xl, this would be done with a `cpuid=’ domain config option).