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
