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 >
