xen-kernel -- x86: Mishandling of SYSCALL singlestep during emulation

The Xen Project reports:

The typical behaviour of singlestepping exceptions is determined at
the start of the instruction, with a #DB trap being raised at the
end of the instruction. SYSCALL (and SYSRET, although we don’t
implement it) behave differently because the typical behaviour
allows userspace to escalate its privilege. (This difference in
behaviour seems to be undocumented.) Xen wrongly raised the
exception based on the flags at the start of the instruction.
Guest userspace which can invoke the instruction emulator can use
this flaw to escalate its privilege to that of the guest kernel.