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

From: Andy Lutomirski <luto@amacapital.net>
To: Kees Cook <keescook@chromium.org>
Cc: Theodore Tso <tytso@google.com>, Willy Tarreau <w@1wt.eu>,
	Dirk Steinmetz <public@rsjtdrjgfuzkfg.com>,
	Michael Kerrisk-manpages <mtk.manpages@gmail.com>,
	Serge Hallyn <serge.hallyn@ubuntu.com>,
	Seth Forshee <seth.forshee@canonical.com>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Linux FS Devel <linux-fsdevel@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	"Eric W . Biederman" <ebiederm@xmission.com>,
	Serge Hallyn <serge.hallyn@canonical.com>,
	"security@kernel.org" <security@kernel.org>
Subject: Re: [RFC] namei: prevent sgid-hardlinks for unmapped gids
Date: Fri, 6 Nov 2015 16:48:26 -0800	[thread overview]
Message-ID: <CALCETrVaB8saXsUrp-qfCAV=ocDfhAuM4PQ8pHjEStS4vDNF6w@mail.gmail.com> (raw)
In-Reply-To: <CAGXu5jJH8VCbHcwLNaoDw5uxGcUzpckT6GJyp1u_wANfURwLhw@mail.gmail.com>

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