After UFFDIO_POISON, there can be two kinds of hugetlb pte markers, either
the POISON one or UFFD_WP one.

Allow change protection to run on a poisoned marker just like !hugetlb
cases, ignoring the marker irrelevant of the permission.

Here the two bits are mutual exclusive.  For example, when install a
poisoned entry it must not be UFFD_WP already (by checking pte_none()
before such install).  And it also means if UFFD_WP is set there must have
no POISON bit set.  It makes sense because UFFD_WP is a bit to reflect
permission, and permissions do not apply if the pte is poisoned and
destined to sigbus.

So here we simply check uffd_wp bit set first, do nothing otherwise.

Attach the Fixes to UFFDIO_POISON work, as before that it should not be
possible to have poison entry for hugetlb (e.g., hugetlb doesn't do swap,
so no chance of swapin errors).

Link: https://lkml.kernel.org/r/20240405231920.1772199-1-peterx@redhat.com
Link: https://lore.kernel.org/r/000000000000920d5e0615602dd1@google.com
Fixes: fc71884a ("mm: userfaultfd: add new UFFDIO_POISON ioctl")
Signed-off-by: Peter Xu <peterx@redhat.com>
Reported-by: syzbot+b07c8ac8eee3d4d8440f@syzkaller.appspotmail.com
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Axel Rasmussen <axelrasmussen@google.com>
Cc: <stable@vger.kernel.org>	[6.6+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>