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

[Andrew Morton <akpm@linux-foundation.org>]

On Thu,  9 Apr 2026 11:28:22 -0400 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.

Please can we have a description of the userspace-visible impact of the
bug.

> Fixes: adef440691ba ("userfaultfd: UFFDIO_MOVE uABI")
> Cc: stable@vger.kernel.org

especially when cc:stable, thanks.

> Signed-off-by: Gregory Price <gourry@gourry.net>
>
> ...
>
> --- 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);
>  

(presently wondering if this is backward compatible)