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Patch series "mm: introduce memfd_secret system call to create "secret" memory areas", v20.

This is an implementation of "secret" mappings backed by a file
descriptor.

The file descriptor backing secret memory mappings is created using a
dedicated memfd_secret system call The desired protection mode for the
memory is configured using flags parameter of the system call.  The mmap()
of the file descriptor created with memfd_secret() will create a "secret"
memory mapping.  The pages in that mapping will be marked as not present
in the direct map and will be present only in the page table of the owning
mm.

Although normally Linux userspace mappings are protected from other users,
such secret mappings are useful for environments where a hostile tenant is
trying to trick the kernel into giving them access to other tenants
mappings.

It's designed to provide the following protections:

* Enhanced protection (in conjunction with all the other in-kernel
  attack prevention...