Messages in this thread | | | Date | Thu, 28 Sep 2023 19:51:18 +0200 | From | David Hildenbrand <> | Subject | Re: [PATCH v2 2/3] userfaultfd: UFFDIO_REMAP uABI |
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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
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