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

From: Kees Cook
Date: Sat Nov 07 2015 - 00:06:48 EST

On Fri, Nov 6, 2015 at 4:48 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> On Fri, Nov 6, 2015 at 4:16 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>> On Fri, Nov 6, 2015 at 4:11 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>>> On Fri, Nov 6, 2015 at 2:30 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>>>> On Fri, Nov 6, 2015 at 1:59 PM, Kees Cook <keescook@xxxxxxxxxxxx> 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@xxxxxxxxxxxxxx> wrote:
>>>>>> On Tue, Nov 3, 2015 at 10:58 PM, Willy Tarreau <w@xxxxxx> 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?

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.

Kees Cook
Chrome OS Security
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