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    SubjectRe: [PATCH v2 1/2] mm/migrate: Fix read-only page got writable when recover pte
    On 10.11.22 21:31, Peter Xu wrote:
    > 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
    > sympton 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. I didn't verify, but I think it'll be
    > the same for mprotect()ed pages and after migration we can miss the sigbus
    > instead.

    I don't think so. mprotect() handling relies on vma->vm_page_prot, which
    is supposed to do the right thing. E.g., map the pte protnone without
    VM_READ/VM_WRITE/....

    >
    > 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. To satisfy the need on the backport, I'm attaching
    > the Fixes tag to the uffd-wp shmem support. Since no one had issue with
    > mprotect, so I assume that's also the kernel version we should start to
    > backport for stable, and we shouldn't need to worry before that.
    >
    > Cc: Andrea Arcangeli <aarcange@redhat.com>
    > Cc: stable@vger.kernel.org
    > Fixes: b1f9e876862d ("mm/uffd: enable write protection for shmem & hugetlbfs")
    > Reported-by: Ives van Hoorne <ives@codesandbox.io>
    > Signed-off-by: Peter Xu <peterx@redhat.com>
    > ---
    > mm/migrate.c | 8 +++++++-
    > 1 file changed, 7 insertions(+), 1 deletion(-)
    >
    > diff --git a/mm/migrate.c b/mm/migrate.c
    > index dff333593a8a..8b6351c08c78 100644
    > --- a/mm/migrate.c
    > +++ b/mm/migrate.c
    > @@ -213,8 +213,14 @@ static bool remove_migration_pte(struct folio *folio,
    > 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);


    Any particular reason why not to simply glue this to pte_swp_uffd_wp(),
    because only that needs special care:

    if (pte_swp_uffd_wp(*pvmw.pte)) {
    pte = pte_wrprotect(pte);
    pte = pte_mkuffd_wp(pte);
    }


    And that would match what actually should have been done in commit
    f45ec5ff16a7 -- only special-case uffd-wp.

    Note that I think there are cases where we have a PTE that was
    !writable, but after migration we can map it writable.


    BTW, does unuse_pte() need similar care?
    new_pte = pte_mkold(mk_pte(page, vma->vm_page_prot));
    if (pte_swp_uffd_wp(*pte))
    new_pte = pte_mkuffd_wp(new_pte);
    set_pte_at(vma->vm_mm, addr, pte, new_pte);

    --
    Thanks,

    David / dhildenb

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