linux-kernel - Re: [PATCH] userns/capability: Add user namespace capability

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Message-ID: <56240599.3050903@nod.at>
Date:	Sun, 18 Oct 2015 22:48:25 +0200
From:	Richard Weinberger <richard@....at>
To:	Tobias Markus <tobias@...lix.eu>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	Andrew Morton <akpm@...uxfoundation.org>,
	Andy Lutomirski <luto@...capital.net>,
	Christoph Lameter <cl@...ux.com>,
	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>,
	LSM <linux-security-module@...r.kernel.org>,
	"open list:ABI/API" <linux-api@...r.kernel.org>,
	linux-man <linux-man@...r.kernel.org>
Subject: Re: [PATCH] userns/capability: Add user namespace capability

Am 18.10.2015 um 22:41 schrieb Tobias Markus:
> On 18.10.2015 22:21, Richard Weinberger wrote:
>> Am 18.10.2015 um 22:13 schrieb Tobias Markus:
>>> On 17.10.2015 22:17, Richard Weinberger wrote:
>>>> On Sat, Oct 17, 2015 at 5:58 PM, Tobias Markus <tobias@...lix.eu> wrote:
>>>>> One question remains though: Does this break userspace executables that
>>>>> expect being able to create user namespaces without priviledge? Since
>>>>> creating user namespaces without CAP_SYS_ADMIN was not possible before
>>>>> Linux 3.8, programs should already expect a potential EPERM upon calling
>>>>> clone. Since creating a user namespace without CAP_SYS_USER_NS would
>>>>> also cause EPERM, we should be on the safe side.
>>>>
>>>> In case of doubt, yes it will break existing software.
>>>> Hiding user namespaces behind CAP_SYS_USER_NS will not magically
>>>> make them secure.
>>>>
>>> The goal is not to make user namespaces secure, but to limit access to
>>> them somewhat in order to reduce the potential attack surface.
>>
>> We have already a framework to reduce the attack surface, seccomp.
>> There is no need to invent new capabilities for every non-trivial
>> kernel feature.
>>
>> I can understand the user namespaces seems scary and had bugs.
>> But which software didn't?
>>
>> Are there any unfixed exploitable bugs in user namespaces in recent kerenls?
>>
>> Thanks,
>> //richard
> 
> Isn't seccomp about setting a per-thread syscall filter? Correct me if
> I'm wrong, but I don't know of any way of using seccomp to globally ban
> the use of clone or unshare with CLONE_NEWUSER except for a few
> whiteliste executables, and that's the idea of this hypothetical capability.

This is correct.
If you want it globally you can still use LSM.

> Sure, there are no known exploitable bugs in recent kernels, but would
> you guarantee that for the next 10 years? Every software has bugs, some
> of them exploitable, no amount of testing can prevent that. I'm not
> paranoid, but on the other hand, why should every Linux user having
> CONFIG_USER_NS enabled have to expose more attack surface than he
> absolutely has to?

And what about all the other kernel features?
I really don't get why you choose user namespaces as your enemy.

> Richard, would you run an Apache HTTP server exposed to the internet on
> your own laptop, without any security precautions? According to your
> reasoning, Apache is surely scary and has many bugs, but every software
> has bugs, right?

This argument is bogus and you know that too.

> I really don't want to introduce a user-facing API change just for the
> fun of it - so if there's any better way to do this, please tell me.

As I said, it really don't see why we should treat user namespaces in a special
way. It is a kernel feature like many others are. If you don't trust it, disable it.

Thanks,
//richard
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