CVE-2024-36881

In the Linux kernel, the following vulnerability has been resolved:
mm/userfaultfd: reset ptes when close() for wr-protected ones Userfaultfd
unregister includes a step to remove wr-protect bits from all the relevant
pgtable entries, but that only covered an explicit UFFDIO_UNREGISTER ioctl,
not a close() on the userfaultfd itself. Cover that too. This fixes a WARN
trace. The only user visible side effect is the user can observe leftover
wr-protect bits even if the user close()ed on an userfaultfd when releasing
the last reference of it. However hopefully that should be harmless, and
nothing bad should happen even if so. This change is now more important
after the recent page-table-check patch we merged in mm-unstable
(446dd9ad37d0 (“mm/page_table_check: support userfault wr-protect
entries”)), as we’ll do sanity check on uffd-wp bits without vma context.
So it’s better if we can 100% guarantee no uffd-wp bit leftovers, to make
sure each report will be valid.