fs/proc/task_mmu: fix loss of young/dirty bits during pagemap scan

make_uffd_wp_pte() was previously doing:

  pte = ptep_get(ptep);
  ptep_modify_prot_start(ptep);
  pte = pte_mkuffd_wp(pte);
  ptep_modify_prot_commit(ptep, pte);

But if another thread accessed or dirtied the pte between the first 2
calls, this could lead to loss of that information.  Since
ptep_modify_prot_start() gets and clears atomically, the following is the
correct pattern and prevents any possible race.  Any access after the
first call would see an invalid pte and cause a fault:

  pte = ptep_modify_prot_start(ptep);
  pte = pte_mkuffd_wp(pte);
  ptep_modify_prot_commit(ptep, pte);

Link: https://lkml.kernel.org/r/20240429114017.182570-1-ryan.roberts@arm.com


Fixes: 52526ca7 ("fs/proc/task_mmu: implement IOCTL to get and optionally clear info about PTEs")
Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Muhammad Usama Anjum <usama.anjum@collabora.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>