linux-kernel - Re: [PATCH resend 1/2] capability: introduce sysctl for controlled user-ns capability whitelist

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Message-ID: <CAF2d9jg__SpwvqwYuiXC2d-WxC2C3wq-o-66=K5+ze+BMFPkdQ@mail.gmail.com>
Date:   Fri, 10 Nov 2017 12:43:47 +0900
From:   Mahesh Bandewar (महेश बंडेवार) 
        <maheshb@...gle.com>
To:     "Serge E. Hallyn" <serge@...lyn.com>
Cc:     Mahesh Bandewar <mahesh@...dewar.net>,
        LKML <linux-kernel@...r.kernel.org>,
        Netdev <netdev@...r.kernel.org>,
        Kernel-hardening <kernel-hardening@...ts.openwall.com>,
        Linux API <linux-api@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        Eric Dumazet <edumazet@...gle.com>,
        David Miller <davem@...emloft.net>
Subject: Re: [PATCH resend 1/2] capability: introduce sysctl for controlled
 user-ns capability whitelist

On Fri, Nov 10, 2017 at 2:30 AM, Serge E. Hallyn <serge@...lyn.com> wrote:
> Quoting Mahesh Bandewar (mahesh@...dewar.net):
>> From: Mahesh Bandewar <maheshb@...gle.com>
>>
>> Add a sysctl variable kernel.controlled_userns_caps_whitelist. This
>
> I understand the arguments in favor of whitelists in most cases for
> security purposes.  But given that you've said the goal here is to
> prevent use of a capability in a user namespace when a CVE has been
> found, a whitelist seems the wrong choice, since
>
> 1. it means that an attacker may through some other means be able
> to add a capability back into the whitelist when you specifically
> wanted to drop it.  With a blacklist, you could say "once a cap has
> been dropped it can never be re-added without rebooting".
> 2. it means by default all capabilities will be denied once the
> switch is pulled which is specifically not what you want in this
> case.
> 3. the admin can't just say "drop CAP_NET_ADMIN", but needs to
> know to echo ~CAP_NET_ADMIN.
>
> Why not make it a blacklist, and once a cap is dropped it can
> never be re-added?
>
Well, I'm not going to deny that blacklist approach would work equally
well but code becomes little simpler when you use the whitelist
approach. especially less complicated when a new capability needs to
be added (not that we add capabilities very often) but that would be
something one would have to pay attention to. However with this
approach I can just the CAP_FULL_SET which is readily available.

Having said that I specifically don't have strong preference in this
regard (whitelist vs. blacklist).

> -serge