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    SubjectRe: [RFC] namei: prevent sgid-hardlinks for unmapped gids
    On Fri, Nov 06, 2015 at 09:05:57PM -0800, Kees Cook wrote:
    > >>>> They're certainly not used early enough -- we need to remove suid when
    > >>>> the page becomes writable via mmap (wp_page_shared), not when
    > >>>> writeback happens, or at least not only when writeback happens.
    > >>>
    > >>> Well, I'm shy about the change there. For example, we don't strip in
    > >>> on open(RDWR), just on write().
    > >>
    > >> I take it back. Hooking wp_page_shared looks expensive. :) Maybe we do
    > >> need to hook the mmap?
    > >
    > > But file_update_time already pokes at the same (or nearby) cachelines,
    > > I think -- why would it be expensive? The whole thing could be
    > > guarded by if (unlikely(is setuid)), right?
    >
    > Yeah, true. I added file_remove_privs calls near all the
    > file_update_time calls, to no effect. Added to wp_page_shared too,
    > nothing. Hmmm.

    Why not put the the should_remove_suid() call in
    filemap_page_mkwrite(), or maybe do_page_mkwrite()?

    Cheers,

    - Ted


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