mm/hugetlb: fix uffd-wp bit lost when unsharing happens

mm/hugetlb: fix uffd-wp bit lost when unsharing happens

When we try to unshare a pinned page for a private hugetlb, uffd-wp bit
can get lost during unsharing.

When above condition met, one can lose uffd-wp bit on the privately mapped
hugetlb page.  It allows the page to be writable even if it should still be
wr-protected.  I assume it can mean data loss.

This should be very rare, only if an unsharing happened on a private
hugetlb page with uffd-wp protected (e.g.  in a child which shares the
same page with parent with UFFD_FEATURE_EVENT_FORK enabled).

When I wrote the reproducer (provided in the last patch) I needed to
use the newest gup_test cmd introduced by David to trigger it because I
don't even know another way to do a proper RO longerm pin.

Besides that, it needs a bunch of other conditions all met:

        (1) hugetlb being mapped privately,
        (2) userfaultfd registered with WP and EVENT_FORK,
        (3) the user app fork()s, then,
        (4) RO longterm pin onto a wr-protected anonymous page.

If it's not impossible to hit in production I'd say extremely rare.

Link: https://lkml.kernel.org/r/20230417195317.898696-3-peterx@redhat.com
Fixes: 166f3ecc0daf ("mm/hugetlb: hook page faults for uffd write protection")
Signed-off-by: Peter Xu <peterx@redhat.com>
Reported-by: Mike Kravetz <mike.kravetz@oracle.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Mika Penttilä <mpenttil@redhat.com>
Cc: Nadav Amit <nadav.amit@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>