Re: [PATCH] userfaultfd: preserve write protection across UFFDIO_MOVE

[Suren Baghdasaryan <surenb@google.com>]

On Thu, Apr 9, 2026 at 8:28 AM Gregory Price <gourry@gourry.net> wrote:
>
> move_present_ptes() unconditionally makes the destination PTE writable,
> dropping uffd-wp write-protection from the source PTE.
>
> The original intent was to follow mremap() behavior, but mremap()'s
> move_ptes() preserves the source write state unconditionally.
>
> Modify uffd to preserve the source write state and check the uffd-wp
> condition of the source before setting writable on the destination.
>
> Fixes: adef440691ba ("userfaultfd: UFFDIO_MOVE uABI")
> Cc: stable@vger.kernel.org
> Signed-off-by: Gregory Price <gourry@gourry.net>

Reviewed-by: Suren Baghdasaryan <surenb@google.com>

> ---
>  mm/userfaultfd.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> index e6dfd5f28acd..783ca68aed88 100644
> --- a/mm/userfaultfd.c
> +++ b/mm/userfaultfd.c
> @@ -1123,7 +1123,10 @@ static long move_present_ptes(struct mm_struct *mm,
>                         orig_dst_pte = pte_mksoft_dirty(orig_dst_pte);
>                 if (pte_dirty(orig_src_pte))
>                         orig_dst_pte = pte_mkdirty(orig_dst_pte);
> -               orig_dst_pte = pte_mkwrite(orig_dst_pte, dst_vma);
> +               if (pte_write(orig_src_pte))
> +                       orig_dst_pte = pte_mkwrite(orig_dst_pte, dst_vma);
> +               if (pte_uffd_wp(orig_src_pte))
> +                       orig_dst_pte = pte_mkuffd_wp(orig_dst_pte);
>                 set_pte_at(mm, dst_addr, dst_pte, orig_dst_pte);
>
>                 src_addr += PAGE_SIZE;
> --
> 2.52.0
>