Re: [RFC] namei: prevent sgid-hardlinks for unmapped gids

[Theodore Ts'o]

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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/