Advisory: XSA-192

Before XSA-192, a bug existed with Xen's handling of task switches into vm86 mode, whereby LDTR got loaded with vm86 attributes.

On Intel hardware, this causes a vmentry failure because of LDTR consistency checks. AMD hardware performs less in the way consistency checking, and ends up loading a real LDT starting at 0, which stays around even after context switching back into the kernel.

This test enters vm86 mode via task switch, exits immediately, context switches into userspace, and checks whether a usable LDT is present at 0.

See also: tests/xsa-192/main.c