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From: Andy Lutomirski <luto@amacapital.net>
Date: Mon, 8 Dec 2014 14:48:54 -0800
Message-ID: <CALCETrXSScp77BUJR5NSTh5-RnEZ9rqELSGJBeEzgdQ-OtohuQ@mail.gmail.com>
Subject: Re: [CFT][PATCH 6/7] userns: Add a knob to disable setgroups on a per
 user namespace basis
To: "Eric W. Biederman" <ebiederm@xmission.com>
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>
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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.

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).

--Andy

> Since these are independent atomic variables yes that ordering issue
> seems to be the case.
>
> For me it was the natural ordering of the checks so I didn't even bother
> to think about what happens when you reorder them.
>
> Eric



-- 
Andy Lutomirski
AMA Capital Management, LLC
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