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From: Andy Lutomirski <luto@amacapital.net>
Date: Mon, 8 Dec 2014 14:21:58 -0800
Message-ID: <CALCETrU-o5mPr1jCaLXDuuF6J2N470zAtx=8Fa-SjF=ZpdE8mQ@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 2:11 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/

> --- 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:

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.

--Andy


-- 
Andy Lutomirski
AMA Capital Management, LLC
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