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

David Hildenbrand
2022-11-15T17:54:18Z

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