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

[Gregory Price <gourry@gourry.net>]

On Sun, Apr 12, 2026 at 11:18:07AM -0700, Andrew Morton wrote:
> 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.
>

Simply:

  UFFDIO_MOVE silently drops write protection from the source PTE when
  moving pages to a destination, leading to missing write-protect faults
  after the page has been moved.

I ran into this while futzing around with some user space management of
VM memory, and expecting a move to continue firing WP faults after.

But Sashiko actually made a useful (though obtuse) observation which
has made me realize _MOVE is actually ambiguous on what to do with
source region UFFD modes.

> > +		if (pte_uffd_wp(orig_src_pte))
> > +			orig_dst_pte = pte_mkuffd_wp(orig_dst_pte);

This line assumes the destination must have intended to be WP, and the
the result is essentially stale uffd wp bits in the opposite case (a
user not intending to carry over WP now carries it over).

tl;dr: this is more of a semantic change than I'd intended, and the
existing tests did not catch it.

The correct solution here is to make a UFFDIO_MOVE_MODE_WP flag to copy
the UFFDIO_COPY_MODE_WP pattern.  Otherwise:

> (presently wondering if this is backward compatible)

Yes, you're right to wonder - this does break backward compatibilty.

Will come back around with UFFDIO_MOVE_MODE_WP.

~Gregory