xen-kernel -- x86 CMPXCHG8B emulation fails to ignore operand size override

The Xen Project reports:

The x86 instruction CMPXCHG8B is supposed to ignore legacy operand
size overrides; it only honors the REX.W override (making it
CMPXCHG16B). So, the operand size is always 8 or 16. When support
for CMPXCHG16B emulation was added to the instruction emulator,
this restriction on the set of possible operand sizes was relied on
in some parts of the emulation; but a wrong, fully general, operand
size value was used for other parts of the emulation. As a result,
if a guest uses a supposedly-ignored operand size prefix, a small
amount of hypervisor stack data is leaked to the guests: a 96 bit
leak to guests running in 64-bit mode; or, a 32 bit leak to other
guests.
A malicious unprivileged guest may be able to obtain sensitive
information from the host.