The patch titled Subject: mm/migrate: fix read-only page got writable when recover pte has been added to the -mm mm-hotfixes-unstable branch. Its filename is mm-migrate-fix-read-only-page-got-writable-when-recover-pte.patch

This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches...

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: Peter Xu peterx@redhat.com Subject: mm/migrate: fix read-only page got writable when recover pte Date: Sun, 13 Nov 2022 19:04:46 -0500

Ives van Hoorne from codesandbox.io reported an issue regarding possible data loss of uffd-wp when applied to memfds on heavily loaded systems. The symptom is some read page got data mismatch from the snapshot child VMs.

Here I can also reproduce with a Rust reproducer that was provided by Ives that keeps taking snapshot of a 256MB VM, on a 32G system when I initiate 80 instances I can trigger the issues in ten minutes.

It turns out that we got some pages write-through even if uffd-wp is applied to the pte.

The problem is, when removing migration entries, we didn't really worry about write bit as long as we know it's not a write migration entry. That may not be true, for some memory types (e.g. writable shmem) mk_pte can return a pte with write bit set, then to recover the migration entry to its original state we need to explicit wr-protect the pte or it'll has the write bit set if it's a read migration entry. For uffd it can cause write-through.

The relevant code on uffd was introduced in the anon support, which is commit f45ec5ff16a7 ("userfaultfd: wp: support swap and page migration", 2020-04-07). However anon shouldn't suffer from this problem because anon should already have the write bit cleared always, so that may not be a proper Fixes target, while I'm adding the Fixes to be uffd shmem support.

Link: https://lkml.kernel.org/r/20221114000447.1681003-2-peterx@redhat.com Fixes: b1f9e876862d ("mm/uffd: enable write protection for shmem & hugetlbfs") Reported-by: Ives van Hoorne ives@codesandbox.io Reviewed-by: Alistair Popple apopple@nvidia.com Tested-by: Ives van Hoorne ives@codesandbox.io Signed-off-by: Peter Xu peterx@redhat.com Cc: Andrea Arcangeli aarcange@redhat.com Cc: Axel Rasmussen axelrasmussen@google.com Cc: Mike Rapoport rppt@linux.vnet.ibm.com Cc: Nadav Amit nadav.amit@gmail.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

mm/migrate.c | 8 +++++++- 1 file changed, 7 insertions(+), 1 deletion(-)

--- a/mm/migrate.c~mm-migrate-fix-read-only-page-got-writable-when-recover-pte +++ a/mm/migrate.c @@ -213,8 +213,14 @@ static bool remove_migration_pte(struct pte = pte_mkdirty(pte); if (is_writable_migration_entry(entry)) pte = maybe_mkwrite(pte, vma); - else if (pte_swp_uffd_wp(*pvmw.pte)) + else + /* NOTE: mk_pte can have write bit set */ + pte = pte_wrprotect(pte); + + if (pte_swp_uffd_wp(*pvmw.pte)) { + WARN_ON_ONCE(pte_write(pte)); pte = pte_mkuffd_wp(pte); + }

if (folio_test_anon(folio) && !is_readable_migration_entry(entry)) rmap_flags |= RMAP_EXCLUSIVE; _

Patches currently in -mm which might be from peterx@redhat.com are

mm-migrate-fix-read-only-page-got-writable-when-recover-pte.patch mm-always-compile-in-pte-markers.patch mm-use-pte-markers-for-swap-errors.patch