Re: [PATCH v2 1/2] mm/migrate: Fix read-only page got writable when recover pte

[David Hildenbrand]

On 15.11.22 18:22, David Hildenbrand wrote:
> > > I consider UFFD-wp a special case: while the default VMA protection might
> > > state that it is writable, you actually want individual PTEs to be
> > > write-protected and have to manually remove the protection.
> > >
> > > softdirty tracking is another special case: however, softdirty tracking is
> > > enabled for the whole VMA. For remove_migration_pte() that should be fine (I
> > > guess) because writenotify is active when the VMA needs to track softdirty
> > > bits, and consequently vma->vm_page_prot has the proper default permissions.
> > >
> > >
> > > I wonder if the following (valid), for example is possible:
> > >
> > >
> > > 1) clear_refs() clears VM_SOFTDIRTY and pte_wrprotect() the pte.
> > > -> writenotify is active and vma->vm_page_prot updated accordingly
> > >
> > > VM_SOFTDIRTY is reset due to VMA merging and vma->vm_page_prot is updated
> > > accordingly. See mmap_region() where we set VM_SOFTDIRTY.
> > >
> > > If you now migrate the (still write-protected in the PTE) page, it was not
> > > writable, but it can be writable on the destination.
> >
> > I didn't even notice merging could work with soft-dirty enabled, that's
> > interesting to know.
> >
> > Yes I think it's possible and I agree it's safe, as VM_SOFTDIRTY is set for
> > the merged vma so afaiu the write bit is safe to set.  We get a bunch of
> > false positives but that's how soft-dirty works.
> >
> > I think the whole problem is easier if we see this at a higher level.
> > You're discussing this from vma pov and it's fair to do so, at least I
> > agree with what you mentioned so far and I can't see anything outside
> > uffd-wp that can be affected.  However, it is also true when you noticed we
> > already have quite a few paragraphs trying to discuss the safety for this
> > and that, that's the part where I think we need justification and it's not
> > that "natural".

Forgot to reply to that part:

No it isn't natural. But sneaking such a change into your fix seems 
wrong. Touching !uffd-wp code should be separate, if we want to do this 
at all (as we discussed, maybe the better/cleaner approach is to 
eliminate writable migration entries if possible).

--
Thanks,

David / dhildenb