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

[David Hildenbrand <david@redhat.com>]

>> +static int remap_anon_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm,
>> +                         struct vm_area_struct *dst_vma,
>> +                         struct vm_area_struct *src_vma,
>> +                         unsigned long dst_addr, unsigned long src_addr,
>> +                         pte_t *dst_pte, pte_t *src_pte,
>> +                         pte_t orig_dst_pte, pte_t orig_src_pte,
>> +                         spinlock_t *dst_ptl, spinlock_t *src_ptl,
>> +                         struct folio *src_folio)
>> +{
>> +       struct anon_vma *dst_anon_vma;
>> +
>> +       double_pt_lock(dst_ptl, src_ptl);
>> +
>> +       if (!pte_same(*src_pte, orig_src_pte) ||
>> +           !pte_same(*dst_pte, orig_dst_pte) ||
>> +           folio_test_large(src_folio) ||
>> +           folio_estimated_sharers(src_folio) != 1) {

^ here you should check PageAnonExclusive. Please get rid of any 
implicit explicit/implcit mapcount checks.

>> +               double_pt_unlock(dst_ptl, src_ptl);
>> +               return -EAGAIN;
>> +       }
>> +
>> +       BUG_ON(!folio_test_anon(src_folio));
>> +
>> +       dst_anon_vma = (void *)dst_vma->anon_vma + PAGE_MAPPING_ANON;
>> +       WRITE_ONCE(src_folio->mapping,
>> +                  (struct address_space *) dst_anon_vma);

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.

>> +       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)

-- 
Cheers,

David / dhildenb