netdev - Re: [PATCHv4 0/2] capability controlled user-namespaces

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Date:   Wed, 3 Jan 2018 21:53:45 -0800
From:   Mahesh Bandewar (महेश बंडेवार) 
        <maheshb@...gle.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Mahesh Bandewar <mahesh@...dewar.net>,
        LKML <linux-kernel@...r.kernel.org>,
        James Morris <james.l.morris@...cle.com>,
        Netdev <netdev@...r.kernel.org>,
        Kernel-hardening <kernel-hardening@...ts.openwall.com>,
        Linux API <linux-api@...r.kernel.org>,
        Linux Security <linux-security-module@...r.kernel.org>,
        Serge Hallyn <serge@...lyn.com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Kees Cook <keescook@...omium.org>,
        Eric Dumazet <edumazet@...gle.com>,
        David Miller <davem@...emloft.net>
Subject: Re: [PATCHv4 0/2] capability controlled user-namespaces

On Wed, Jan 3, 2018 at 8:44 AM, Eric W. Biederman <ebiederm@...ssion.com> wrote:
> Mahesh Bandewar <mahesh@...dewar.net> writes:
>
>> From: Mahesh Bandewar <maheshb@...gle.com>
>>
>> TL;DR version
>> -------------
>> Creating a sandbox environment with namespaces is challenging
>> considering what these sandboxed processes can engage into. e.g.
>> CVE-2017-6074, CVE-2017-7184, CVE-2017-7308 etc. just to name few.
>> Current form of user-namespaces, however, if changed a bit can allow
>> us to create a sandbox environment without locking down user-
>> namespaces.
>
> In other conversations it appears it has been pointed out that user
> namespaces are not necessarily safe under no_new_privs.  In theory
> user namespaces should be safe but in practice not so much.
>
> So let me ask.  Would your concerns be addressed if we simply made
> creation and joining of user namespaces impossible in a no_new_privs
> sandbox?
>
Isn't this another form of locking down user-ns similar to setting per
user-ns sysctl max_userns = 0?

Having said that, not allowing processes to create and/or attach
user-namespaces is going to be problematic and possibly a regression.
This (current) patchset doesn't do that. It allows users to create
user-ns's of any depth and number permitted by per-ns max_userns
sysctl. However one can decide what to take-off and what to leave in
terms of capabilities for the sandbox environment.

--mahesh..

> Eric
>
[...]