+ fs-proc-task_mmu-fix-loss-of-young-dirty-bits-during-pagemap-scan.patch added to mm-hotfixes-unstable branch

+ fs-proc-task_mmu-fix-loss-of-young-dirty-bits-during-pagemap-scan.patch added to mm-hotfixes-unstable branch

[Andrew Morton]

The patch titled
     Subject: fs/proc/task_mmu: fix loss of young/dirty bits during pagemap scan
has been added to the -mm mm-hotfixes-unstable branch.  Its filename is
     fs-proc-task_mmu-fix-loss-of-young-dirty-bits-during-pagemap-scan.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/fs-proc-task_mmu-fix-loss-of-young-dirty-bits-during-pagemap-scan.patch

This patch will later appear in the mm-hotfixes-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Ryan Roberts <ryan.roberts@arm.com>
Subject: fs/proc/task_mmu: fix loss of young/dirty bits during pagemap scan
Date: Mon, 29 Apr 2024 12:40:17 +0100

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: 52526ca7fdb9 ("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>
---

 fs/proc/task_mmu.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/fs/proc/task_mmu.c~fs-proc-task_mmu-fix-loss-of-young-dirty-bits-during-pagemap-scan
+++ a/fs/proc/task_mmu.c
@@ -1825,7 +1825,7 @@ static void make_uffd_wp_pte(struct vm_a
 		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);
_

Patches currently in -mm which might be from ryan.roberts@arm.com are

fs-proc-task_mmu-fix-loss-of-young-dirty-bits-during-pagemap-scan.patch
fs-proc-task_mmu-fix-uffd-wp-confusion-in-pagemap_scan_pmd_entry.patch
selftests-mm-soft-dirty-should-fail-if-a-testcase-fails.patch
mm-fix-race-between-__split_huge_pmd_locked-and-gup-fast.patch