On Wed, 6 May 2020 15:38:16 -0400
Peter Xu [off-list ref] wrote:

If this is going to be added... I am thinking whether it should be easier to
add another value for unprivileged_userfaultfd, rather than a new sysctl. E.g.:

  "0": unprivileged userfaultfd forbidden
  "1": unprivileged userfaultfd allowed (both user/kernel faults)
  "2": unprivileged userfaultfd allowed (only user faults)

Because after all unprivileged_userfaultfd_user_mode_only will be meaningless
(iiuc) if unprivileged_userfaultfd=0.  The default value will also be the same
as before ("1") then.

It occurs to me to wonder whether this interface should also let an admin
block *privileged* user from handling kernel-space faults?  In a
secure-boot/lockdown setting, this could be a hardening measure that keeps
a (somewhat) restricted root user from expanding their privilege...?

jon