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

[Kees Cook]

On Tue, Nov 10, 2015 at 7:08 AM, Jan Kara <jack@xxxxxxx> wrote:
> On Sat 07-11-15 21:02:06, Ted Tso wrote:
>> 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()?
>
> page_mkwrite() callbacks are IMHO the right place for this check (and
> change).  Just next to file_update_time() call. You get proper filesystem

Should file_update_time() just be modified to include
file_remove_privs()? They seem to regularly go together.

> freezing protection etc. As Ted properly mentions filemap_page_mkwrite() is
> one place you want to hook into but quite a few filesystems (most notably
> ext4, xfs, btrfs) overload ->page_mkwrite() callbacks to their own
> functions so each filesystem that does this needs to be updated
> separately...

Depending on each filesystem to do this correctly seems like a
mistake. I was surprised that my attempts (via file_update_time and
wp_page_shared) didn't work. Any other suggestions?

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security
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