why pager in utcb

Hi,

I studied the pistachio kernel and found the pager thread id of a thread is
saved in its utcb. Why not save the pager attribute in tcbs as the
scheduler?

IMHO, it may cause some security problems if a thread can freely change its
pager. For example envn if we prohibit the direct IPC between two threads,
they may still be possible to exploit page fault IPC to achieve
communication. And we may have to check pagefault IPC to avoid such
problems, which I think is not a good idea for performance consideration.

If I modify the kernel so that threads' pagers are stored in tcbs and can
only be altered through thread_control privileged system call, whether
current applications (e.g. kenge and iguana) on pistachio will be affected?
How severe the result is? Could you give some thoughts on the demand for
unprivileged pager modification?

Regards,

Wei