From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1033919AbbKFV7x (ORCPT <rfc822;w@1wt.eu>);
	Fri, 6 Nov 2015 16:59:53 -0500
Received: from mail-ig0-f176.google.com ([209.85.213.176]:36259 "EHLO
	mail-ig0-f176.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1030287AbbKFV7v (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Fri, 6 Nov 2015 16:59:51 -0500
MIME-Version: 1.0
In-Reply-To: <CALCETrVKuAW=qYKsB_Dkmq2LDwSV1sdP8sKbuDcGzye3zn_e2A@mail.gmail.com>
References: <CALCETrUPyFMAk9uxJ8+hg0YoMN=mTM3q9jAT7zki1g9uuWtVkg@mail.gmail.com>
	<1446511187-9131-1-git-send-email-public@rsjtdrjgfuzkfg.com>
	<CAGXu5j+i6PZNMwmseZc8G6V_5=5TK=dXP=bzLteK705bLdUU2A@mail.gmail.com>
	<20151104002132.010ccd1d@rsjtdrjgfuzkfg.com>
	<CAGXu5jKCCq4hs_nerQYMnC1frA3qKkCxxQTjpS2gw-7dPZAX+w@mail.gmail.com>
	<20151104065820.GF21740@1wt.eu>
	<CALCETrVKuAW=qYKsB_Dkmq2LDwSV1sdP8sKbuDcGzye3zn_e2A@mail.gmail.com>
Date: Fri, 6 Nov 2015 13:59:50 -0800
X-Google-Sender-Auth: 5zqMqx3Q92tQBv_DZYwVpfkNTy0
Message-ID: <CAGXu5jJ3yKaeWJb1b6aJTNJMrX1f+vTabMn3ATuz_cuksuXDog@mail.gmail.com>
Subject: Re: [RFC] namei: prevent sgid-hardlinks for unmapped gids
From: Kees Cook <keescook@chromium.org>
To: Andy Lutomirski <luto@amacapital.net>, Theodore Tso <tytso@google.com>
Cc: 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>
Content-Type: text/plain; charset=UTF-8
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

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?

>>
>>> Should the
>>> mmap MAP_SHARED-write trigger the loss of the set-gid bit too? While
>>> holding the file open with either open or mmap, I get a Text-in-use
>>> error, so I would kind of expect the same behavior between either
>>> close() and munmap(). I wonder if this is a bug, and if so, then your
>>> link patch is indeed useful again. :)
>>
>> I don't see how this could be done with mmap(). Maybe we have a way to know
>> when the first write is performed via this path, I have no idea.
>
> do_wp_page might be a decent bet.

Or wp_page_shared? Can we get back to a file from the mm at that point?

-Kees

-- 
Kees Cook
Chrome OS Security