From: ebiederm@xmission.com (Eric W. Biederman)
To: Andy Lutomirski <luto@amacapital.net>
Cc: Linux Containers <containers@lists.linux-foundation.org>,
        Josh Triplett <josh@joshtriplett.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Kees Cook <keescook@chromium.org>,
        Michael Kerrisk-manpages <mtk.manpages@gmail.com>,
        Linux API <linux-api@vger.kernel.org>,
        linux-man <linux-man@vger.kernel.org>,
        "linux-kernel\@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        LSM <linux-security-module@vger.kernel.org>,
        Casey Schaufler <casey@schaufler-ca.com>,
        "Serge E. Hallyn" <serge@hallyn.com>,
        Richard Weinberger <richard@nod.at>,
        Kenton Varda <kenton@sandstorm.io>, stable <stable@vger.kernel.org>
References: <52e0643bd47b1e5c65921d6e00aea1f724bb510a.1417281801.git.luto@amacapital.net>
	<87h9xez20g.fsf@x220.int.ebiederm.org>
	<CALCETrXOz4C7Tu8mggBtR=k47ZmkuAhinVUxWJSFyS1Ep0HvRw@mail.gmail.com>
	<87mw75ygwp.fsf@x220.int.ebiederm.org>
	<CALCETrVfO4sBdZcQiZXsofPZMj7pqKeVbX+4g3dAj6WjUca+1w@mail.gmail.com>
	<87fvcxyf28.fsf_-_@x220.int.ebiederm.org>
	<874mtdyexp.fsf_-_@x220.int.ebiederm.org>
	<CALCETrXyC7XPaqj6oe-TmyypOVc_CkZbF6UAAx8YfkyD=gEMOQ@mail.gmail.com>
	<87a935u3nj.fsf@x220.int.ebiederm.org>
	<CALCETrXkEOiyzpvqtXtk1f4sL+M1Q-Y6rV=K91ez3yv2nb4Y0Q@mail.gmail.com>
	<87388xodlj.fsf@x220.int.ebiederm.org>
	<CALCETrXWx2-ZejEHmOi7aSoF-qJMRGR5yseeMhuurZwJRrQbUg@mail.gmail.com>
	<87h9x5re41.fsf_-_@x220.int.ebiederm.org>
	<87mw6xpzb0.fsf_-_@x220.int.ebiederm.org>
	<CALCETrU-o5mPr1jCaLXDuuF6J2N470zAtx=8Fa-SjF=ZpdE8mQ@mail.gmail.com>
	<87ppbtn4mv.fsf@x220.int.ebiederm.org>
	<CALCETrXSScp77BUJR5NSTh5-RnEZ9rqELSGJBeEzgdQ-OtohuQ@mail.gmail.com>
Date: Mon, 08 Dec 2014 17:30:07 -0600
In-Reply-To: <CALCETrXSScp77BUJR5NSTh5-RnEZ9rqELSGJBeEzgdQ-OtohuQ@mail.gmail.com>
	(Andy Lutomirski's message of "Mon, 8 Dec 2014 14:48:54 -0800")
Message-ID: <87a92xn2io.fsf@x220.int.ebiederm.org>
User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/24.3 (gnu/linux)
MIME-Version: 1.0
Content-Type: text/plain
Subject: Re: [CFT][PATCH 6/7] userns: Add a knob to disable setgroups on a per user namespace basis
Sender: linux-kernel-owner@vger.kernel.org

Andy Lutomirski <luto@amacapital.net> writes:

> On Mon, Dec 8, 2014 at 2:44 PM, Eric W. Biederman <ebiederm@xmission.com> wrote:
>> Andy Lutomirski <luto@amacapital.net> writes:
>>
>>> On Mon, Dec 8, 2014 at 2:11 PM, Eric W. Biederman <ebiederm@xmission.com> wrote:
>>>>
>>>> - Expose the knob to user space through a proc file /proc/<pid>/setgroups
>>>>
>>>>   A value of 0 means the setgroups system call is disabled in the
>>>
>>> "deny"
>>>
>>>>   current processes user namespace and can not be enabled in the
>>>>   future in this user namespace.
>>>>
>>>>   A value of 1 means the segtoups system call is enabled.
>>>>
>>>
>>> "allow"
>>>
>>>> - Descedent user namespaces inherit the value of setgroups from
>>>
>>> s/Descedent/Descendent/
>>
>> Bah.  I updated everything but the changelog comment.
>>
>>>> --- a/kernel/groups.c
>>>> +++ b/kernel/groups.c
>>>> @@ -222,6 +222,7 @@ bool may_setgroups(void)
>>>>          * the user namespace has been established.
>>>>          */
>>>>         return userns_gid_mappings_established(user_ns) &&
>>>> +               userns_setgroups_allowed(user_ns) &&
>>>>                 ns_capable(user_ns, CAP_SETGID);
>>>>  }
>>>
>>> Can you add a comment explaining the ordering?  For example:
>>
>> I need to think on what I can say to make it clear.
>> Perhaps: /* Careful the order of these checks is important. */
>>
>>> We need to check for a gid mapping before checking setgroups_allowed
>>> because an unprivileged user can create a userns with setgroups
>>> allowed, then disallow setgroups and add a mapping.  If we check in
>>> the opposite order, then we have a race: we could see that setgroups
>>> is allowed before the user clears the bit and then see that there is a
>>> gid mapping after the other thread is done.
>>
>
> This text was actually my suggested comment text.

Now I see.

> If you put smp_rmb() in this function with a comment like that, then I
> think it will all make sense and be obviously correct (even with most
> of the other barriers removed).

Right.

Given that we have to be careful when using these things anyway what
I was hoping to achieve with the barriers appears impossible, and
confusing so I will see about just adding barriers where we need them
for real.  Sigh.

Eric
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