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

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commit c70dce4982 upstream.

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: 52526ca7fd ("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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Ryan Roberts 2024-04-29 12:40:17 +01:00 committed by Greg Kroah-Hartman
parent 3e7b3519b7
commit 74b3d66f91
1 changed files with 1 additions and 1 deletions
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2
fs/proc/task_mmu.c
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@ -1826,7 +1826,7 @@ static void make_uffd_wp_pte(struct vm_area_struct *vma,
pte_t old_pte;
old_pte = ptep_modify_prot_start(vma, addr, pte);
ptent = pte_mkuffd_wp(ptent);
ptent = pte_mkuffd_wp(old_pte);
ptep_modify_prot_commit(vma, addr, pte, old_pte, ptent);
} else if (is_swap_pte(ptent)) {
ptent = pte_swp_mkuffd_wp(ptent);