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

[Suren Baghdasaryan]

On Thu, Sep 28, 2023 at 10:15 AM David Hildenbrand <david@xxxxxxxxxx> 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.
Thanks!

>
> --
> Cheers,
>
> David / dhildenb
>