xen-kernel -- x86 HVM: Overflow of sh_ctxt->seg_reg[]

The Xen Project reports:

x86 HVM guests running with shadow paging use a subset of the x86
emulator to handle the guest writing to its own pagetables. There
are situations a guest can provoke which result in exceeding the
space allocated for internal state.
A malicious HVM guest administrator can cause Xen to fail a bug
check, causing a denial of service to the host.