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

Suren Baghdasaryan
2023-09-28T20:11:40Z

On Thu, Sep 28, 2023 at 11:32 AM Suren Baghdasaryan <surenb@google.com> wrote:
>
> On Thu, Sep 28, 2023 at 10:15 AM David Hildenbrand <david@redhat.com> wrote:
> >
> > 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 :)
>
> Ack.
>
> >
> > >
> > >>
> > >>>> + 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.
>
> Let me look closer into these cases.
> I'll also double-check if we need to support uffd_remap for R/O vmas.
> I assumed we do but I actually never checked.

Ok, I confirmed that we don't need remapping or R/O areas. So, I can
use can_change_pte_writable() and keep things simple. Does that sound
good?

> Thanks!
>
> >
> > --
> > Cheers,
> >
> > David / dhildenb
> >