FreeBSD -- Local privilege escalation in IRET handler

Problem Description:
If the kernel-mode IRET instruction generates an #SS or
#NP exception, but the exception handler does not properly
ensure that the right GS register base for kernel is reloaded,
the userland GS segment may be used in the context of the
kernel exception handler.
Impact:
By causing an IRET with #SS or #NP exceptions, a local
attacker can cause the kernel to use an arbitrary GS base,
which may allow escalated privileges or panic the system.