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: <CALCETrU-o5mPr1jCaLXDuuF6J2N470zAtx=8Fa-SjF=ZpdE8mQ@mail.gmail.com>
Date:	Mon, 8 Dec 2014 14:21:58 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
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@...r.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

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/

> --- 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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