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Date: Mon, 30 Nov 2015 12:14:41 -0800
From: Kees Cook <>
To: Richard Weinberger <>, "Eric W. Biederman" <>
Cc: "" <>, 
	"" <>, 
	"" <>,, 
	Stephen Hemminger <>
Subject: Re: user controllable usermodehelper in br_stp_if.c

On Sun, Nov 29, 2015 at 2:43 PM, Richard Weinberger <> 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 Cook
Chrome OS & Brillo Security

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