Re: [PATCH v2 2/3] userfaultfd: UFFDIO_REMAP uABI

[David Hildenbrand]

On 27.09.23 20:25, Suren Baghdasaryan wrote:
> > I have some cleanups pending for page_move_anon_rmap(), that moves the
> > SetPageAnonExclusive hunk out. Here we should be using
> > page_move_anon_rmap() [or rather, folio_move_anon_rmap() after my cleanups]
> >
> > I'll send them out soonish.
>
> Should I keep this as is in my next version until you post the
> cleanups? I can add a TODO comment to convert it to
> folio_move_anon_rmap() once it's ready.

You should just be able to use page_move_anon_rmap() and whatever gets 
in first cleans it up :)

> > > > +       WRITE_ONCE(src_folio->index, linear_page_index(dst_vma,
> > > > +                                                     dst_addr)); >> +
> > > > +       orig_src_pte = ptep_clear_flush(src_vma, src_addr, src_pte);
> > > > +       orig_dst_pte = mk_pte(&src_folio->page, dst_vma->vm_page_prot);
> > > > +       orig_dst_pte = maybe_mkwrite(pte_mkdirty(orig_dst_pte),
> > > > +                                    dst_vma);
> > >
> > > I think there's still a theoretical issue here that you could fix by
> > > checking for the AnonExclusive flag, similar to the huge page case.
> > >
> > > Consider the following scenario:
> > >
> > > 1. process P1 does a write fault in a private anonymous VMA, creating
> > > and mapping a new anonymous page A1
> > > 2. process P1 forks and creates two children P2 and P3. afterwards, A1
> > > is mapped in P1, P2 and P3 as a COW page, with mapcount 3.
> > > 3. process P1 removes its mapping of A1, dropping its mapcount to 2.
> > > 4. process P2 uses vmsplice() to grab a reference to A1 with get_user_pages()
> > > 5. process P2 removes its mapping of A1, dropping its mapcount to 1.
> > >
> > > If at this point P3 does a write fault on its mapping of A1, it will
> > > still trigger copy-on-write thanks to the AnonExclusive mechanism; and
> > > this is necessary to avoid P3 mapping A1 as writable and writing data
> > > into it that will become visible to P2, if P2 and P3 are in different
> > > security contexts.
> > >
> > > But if P3 instead moves its mapping of A1 to another address with
> > > remap_anon_pte() which only does a page mapcount check, the
> > > maybe_mkwrite() will directly make the mapping writable, circumventing
> > > the AnonExclusive mechanism.
> >
> > Yes, can_change_pte_writable() contains the exact logic when we can turn
> > something easily writable even if it wasn't writable before. which
> > includes that PageAnonExclusive is set. (but with uffd-wp or softdirty
> > tracking, there is more to consider)
>
> For uffd_remap can_change_pte_writable() would fail it VM_WRITE is not
> set, but we want remapping to work for RO memory as well. Are you

In a VMA without VM_WRITE you certainly wouldn't want to make PTEs 
writable :) That's why that function just does a sanity check that it is 
not called in strange context. So one would only call it if VM_WRITE is set.

> saying that a PageAnonExclusive() check alone would not be enough
> here?

There are some interesting questions to ask here:

1) What happens if the old VMA has VM_SOFTDIRTY set but the new one not? 
You most probably have to mark the PTE softdirty and not make it writable.

2) VM_UFFD_WP requires similar care I assume? Peter might know.

--
Cheers,

David / dhildenb