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

[David Hildenbrand]

On 28.09.23 19:21, Peter Xu wrote:
> On Thu, Sep 28, 2023 at 07:05:40PM +0200, David Hildenbrand wrote:
> > As described as reply to v1, without fork() and KSM, the PAE bit should
> > stick around. If that's not the case, we should investigate why.
> >
> > If we ever support the post-fork case (which the comment above remap_pages()
> > excludes) we'll need good motivation why we'd want to make this
> > overly-complicated feature even more complicated.
>
> The problem is DONTFORK is only a suggestion, but not yet restricted.  If
> someone reaches on top of some !PAE page on src it'll never gonna proceed
> and keep failing, iiuc.

Yes. It won't work if you fork() and not use DONTFORK on the src VMA. We 
should document that as a limitation.

For example, you could return an error to the user that can just call 
UFFDIO_COPY. (or to the UFFDIO_COPY from inside uffd code, but that's 
probably ugly as well).

> do_wp_page() doesn't have that issue of accuracy only because one round of
> CoW will just allocate a new page with PAE set guaranteed, which is pretty
> much self-heal and unnoticed.

Yes. But it might have to copy, at which point the whole optimization of 
remap is gone :)

> So it'll be great if we can have similar self-heal way for PAE.  If not, I
> think it's still fine we just always fail on !PAE src pages, but then maybe
> we should let the user know what's wrong, e.g., the user can just forgot to
> apply DONTFORK then forked.  And then the user hits error and don't know
> what happened.  Probably at least we should document it well in man pages.
Yes, exactly.

> Another option can be we keep using folio_mapcount() for pte, and another
> helper (perhaps: _nr_pages_mapped==COMPOUND_MAPPED && _entire_mapcount==1)
> for thp.  But I know that's not ideal either.

As long as we only set the pte writable if PAE is set, we're good from a 
CVE perspective. The other part is just simplicity of avoiding all these 
mapcount+swapcount games where possible.

(one day folio_mapcount() might be faster -- I'm still working on that 
patch in the bigger picture of handling PTE-mapped THP better)

--
Cheers,

David / dhildenb