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    SubjectRe: [RFC] namei: prevent sgid-hardlinks for unmapped gids
    On Fri, Nov 6, 2015 at 4:16 PM, Kees Cook <keescook@chromium.org> wrote:
    > On Fri, Nov 6, 2015 at 4:11 PM, Kees Cook <keescook@chromium.org> wrote:
    >> On Fri, Nov 6, 2015 at 2:30 PM, Andy Lutomirski <luto@amacapital.net> wrote:
    >>> On Fri, Nov 6, 2015 at 1:59 PM, Kees Cook <keescook@chromium.org> wrote:
    >>>> Adding Ted, who might know how this all hooks together. (The context
    >>>> is that a write() or truncate() on a setgid file clears the setgid,
    >>>> but mmap writes don't.)
    >>>>
    >>>> On Wed, Nov 4, 2015 at 9:59 AM, Andy Lutomirski <luto@amacapital.net> wrote:
    >>>>> On Tue, Nov 3, 2015 at 10:58 PM, Willy Tarreau <w@1wt.eu> wrote:
    >>>>>> On Tue, Nov 03, 2015 at 03:29:55PM -0800, Kees Cook wrote:
    >>>>>>> Using "write" does kill the set-gid bit. I haven't looked at
    >>>>>>> why.
    >>>>>>> Al or anyone else, is there a meaningful distinction here?
    >>>>>>
    >>>>>> I remember this one, I got caught once while trying to put a shell into
    >>>>>> a suid-writable file to get some privileges someone forgot to offer me :-)
    >>>>>>
    >>>>>> It's done by should_remove_suid() which is called upon write() and truncate().
    >>>>
    >>>> file_remove_privs() seems to be the right entry point.
    >>>> __generic_file_write_iter in mm/filemap.c calls it, though. Are these
    >>>> callbacks not used for mmap writes?
    >>>
    >>> 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?

    --Andy


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