linux-kernel - Re: user controllable usermodehelper in br_stp

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Date:	Mon, 30 Nov 2015 12:14:41 -0800
From:	Kees Cook <keescook@...omium.org>
To:	Richard Weinberger <richard@....at>,
	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"kernel-hardening@...ts.openwall.com" 
	<kernel-hardening@...ts.openwall.com>,
	bridge@...ts.linux-foundation.org,
	Stephen Hemminger <stephen@...workplumber.org>
Subject: Re: user controllable usermodehelper in br_stp_if.c

On Sun, Nov 29, 2015 at 2:43 PM, Richard Weinberger <richard@....at> wrote:
> Hi!
>
> By spawning new network and user namesapces an unprivileged user
> is able to execute /sbin/bridge-stp within the initial mount namespace
> with global root rights.
> While this cannot directly be used to break out of a container or gain
> global root rights it could be used by exploit writers as valuable building block.
>
> e.g.
> $ unshare -U -r -n /bin/sh
> $ brctl addbr br0
> $ brctl stp br0 on # this will execute /sbin/bridge-stp
>
> As this mechanism clearly cannot work with containers and seems to be legacy code
> I suggest not calling call_usermodehelper() at all if we're not in the initial user namespace.
> What do you think?

I'm not familiar with how bridge-stp is expected to operate with a
network namespace, but if it's meaningless, then yeah, that seems like
a reasonable change. Can you send a patch? (Also, if it's legacy code,
maybe it could be turned off entirely, not just for containers?)

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security
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