linux-kernel - Re: [CFT][PATCH 6/7] userns: Add a knob to disable setgroups on a per user namespace basis

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Message-ID: <87a92xn2io.fsf@x220.int.ebiederm.org>
Date:	Mon, 08 Dec 2014 17:30:07 -0600
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Linux Containers <containers@...ts.linux-foundation.org>,
	Josh Triplett <josh@...htriplett.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Kees Cook <keescook@...omium.org>,
	Michael Kerrisk-manpages <mtk.manpages@...il.com>,
	Linux API <linux-api@...r.kernel.org>,
	linux-man <linux-man@...r.kernel.org>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
	LSM <linux-security-module@...r.kernel.org>,
	Casey Schaufler <casey@...aufler-ca.com>,
	"Serge E. Hallyn" <serge@...lyn.com>,
	Richard Weinberger <richard@....at>,
	Kenton Varda <kenton@...dstorm.io>,
	stable <stable@...r.kernel.org>
Subject: Re: [CFT][PATCH 6/7] userns: Add a knob to disable setgroups on a per user namespace basis

Andy Lutomirski <luto@...capital.net> writes:

> On Mon, Dec 8, 2014 at 2:44 PM, Eric W. Biederman <ebiederm@...ssion.com> wrote:
>> Andy Lutomirski <luto@...capital.net> writes:
>>
>>> On Mon, Dec 8, 2014 at 2:11 PM, Eric W. Biederman <ebiederm@...ssion.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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