linux-kernel - Re: [PATCH v2 00/10] userns: sysctl limits for namespaces

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Message-ID: <CAGXu5j+ZtZAFpW0ajnnBTRZk17ez6XYF=meEJY_rF7wwgyZB5Q@mail.gmail.com>
Date:	Tue, 26 Jul 2016 09:52:43 -0700
From:	Kees Cook <keescook@...omium.org>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>,
	Linux Containers <containers@...ts.linux-foundation.org>,
	Andy Lutomirski <luto@...capital.net>,
	Jann Horn <jann@...jh.net>, Nikolay Borisov <kernel@...p.com>,
	"Serge E. Hallyn" <serge@...lyn.com>,
	Seth Forshee <seth.forshee@...onical.com>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	Network Development <netdev@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH v2 00/10] userns: sysctl limits for namespaces

On Tue, Jul 26, 2016 at 8:06 AM, Eric W. Biederman
<ebiederm@...ssion.com> wrote:
> "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com> writes:
>
>> Hello Eric,
>>
>> I realized I had a question after the last mail.
>>
>> On 07/21/2016 06:39 PM, Eric W. Biederman wrote:
>>>
>>> This patchset addresses two use cases:
>>> - Implement a sane upper bound on the number of namespaces.
>>> - Provide a way for sandboxes to limit the attack surface from
>>>   namespaces.
>>
>> Can you say more about the second point? What exactly is the
>> problem that is being addressed, and how does the patch series
>> address it? (It would be good to have those details in the
>> revised commit message...)
>
> At some point it was reported that seccomp was not sufficient to disable
> namespace creation.  I need to go back and look at that claim to see
> which set of circumstances that was referring to.  Seccomp doesn't stack
> so I can see why it is an issue.

seccomp does stack. The trouble usually comes from a perception that
seccomp overhead is not trivial, so setting a system-wide policy is a
bit of a large hammer for such a limitiation. Also, at the time,
seccomp could be bypasses with ptrace, but this (as of v4.8) is no
longer true.

> The general problem is that namespaces by their nature (and especially
> in combination with the user namespaces) allow unprivileged users to use
> more of the kernel than a user would have access to without them.  This
> in turn allows malicious users more kernel calls they can use in attempt
> to find an exploitable bug.
>
> So if you are building a sandbox/chroot jail/chromium tab or anything
> like that and you know you won't be needing a kernel feature having an
> easy way to disable the feature is useful for making the kernel
> marginally more secure, as certain attack vectors are no longer
> possible.

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security