From: Christian Brauner <christian.brauner@canonical•com>
To: "Mahesh Bandewar (महेश बंडेवार)" <maheshb@google•com>
Cc: "Serge E. Hallyn" <serge@hallyn•com>,
Boris Lukashev <blukashev@sempervictus•com>,
Daniel Micay <danielmicay@gmail•com>,
Mahesh Bandewar <mahesh@bandewar•net>,
LKML <linux-kernel@vger•kernel.org>,
Netdev <netdev@vger•kernel.org>,
Kernel-hardening <kernel-hardening@lists•openwall.com>,
Linux API <linux-api@vger•kernel.org>,
Kees Cook <keescook@chromium•org>,
"Eric W . Biederman" <ebiederm@xmission•com>,
Eric Dumazet <edumazet@google•com>,
David Miller <davem@davemloft•net>
Subject: Re: [kernel-hardening] Re: [PATCH resend 2/2] userns: control capabilities of some user namespaces
Date: Wed, 8 Nov 2017 20:02:25 +0100 [thread overview]
Message-ID: <20171108190223.vdkyepcaegmub6le@gmail.com> (raw)
In-Reply-To: <CAF2d9jiwu3Ss7Dtem8qSptKgMc0Mc_o8MAAJkWM5CwJaFsbrcg@mail.gmail.com>
On Wed, Nov 08, 2017 at 03:09:59AM -0800, Mahesh Bandewar (महेश बंडेवार) wrote:
> Sorry folks I was traveling and seems like lot happened on this thread. :p
>
> I will try to response few of these comments selectively -
>
> > The thing that makes me hesitate with this set is that it is a
> > permanent new feature to address what (I hope) is a temporary
> > problem.
> I agree this is permanent new feature but it's not solving a temporary
> problem. It's impossible to assess what and when new vulnerability
> that could show up. I think Daniel summed it up appropriately in his
> response
>
> > Seems like there are two naive ways to do it, the first being to just
> > look at all code under ns_capable() plus code called from there. It
> > seems like looking at the result of that could be fruitful.
> This is really hard. The main issue that there were features designed
> and developed before user-ns days with an assumption that unprivileged
> users will never get certain capabilities which only root user gets.
> Now that is not true anymore with user-ns creation with mapping root
> for any process. Also at the same time blocking user-ns creation for
> eveyone is a big-hammer which is not needed too. So it's not that easy
> to just perform a code-walk-though and correct those decisions now.
>
> > It seems to me that the existing control in
> > /proc/sys/kernel/unprivileged_userns_clone might be the better duct tape
> > in that case.
> This solution is essentially blocking unprivileged users from using
> the user-namespaces entirely. This is not really a solution that can
> work. The solution that this patch-set adds allows unprivileged users
> to create user-namespaces. Actually the proposed solution is more
> fine-grained approach than the unprivileged_userns_clone solution
> since you can selectively block capabilities rather than completely
> blocking the functionality.
I've been talking to Stéphane today about this and we should also keep in mind
that we have:
chb@conventiont|~
> ls -al /proc/sys/user/
total 0
dr-xr-xr-x 1 root root 0 Nov 6 23:32 .
dr-xr-xr-x 1 root root 0 Nov 2 22:13 ..
-rw-r--r-- 1 root root 0 Nov 8 19:48 max_cgroup_namespaces
-rw-r--r-- 1 root root 0 Nov 8 19:48 max_inotify_instances
-rw-r--r-- 1 root root 0 Nov 8 19:48 max_inotify_watches
-rw-r--r-- 1 root root 0 Nov 8 19:48 max_ipc_namespaces
-rw-r--r-- 1 root root 0 Nov 8 19:48 max_mnt_namespaces
-rw-r--r-- 1 root root 0 Nov 8 19:48 max_net_namespaces
-rw-r--r-- 1 root root 0 Nov 8 19:48 max_pid_namespaces
-rw-r--r-- 1 root root 0 Nov 8 19:48 max_user_namespaces
-rw-r--r-- 1 root root 0 Nov 8 19:48 max_uts_namespaces
These files allow you to limit the number of namespaces that can be created
*per namespace* type. So let's say your system runs a bunch of user namespaces
you can do:
chb@conventiont|~
> echo 0 > /proc/sys/user/max_user_namespaces
So that the next time you try to create a user namespaces you'd see:
chb@conventiont|~
> unshare -U
unshare: unshare failed: No space left on device
So there's not even a need to upstream a new sysctl since we have ways of
blocking this.
Also I'd like to point out that a lot of capability checks and actual security
vulnerabilities are associated with CAP_SYS_ADMIN. So what you likely want to do
is block CAP_SYS_ADMIN in user namespaces but at this point they become
basically useless for a lot of interesting use cases. In addition, this patch
would add another layer of complexity that is - imho - not really warranted
given what we already have. The relationship between capabilities and user
namespaces should stay as simply as possible so that it stays maintaineable.
User namespaces already introduce a proper layer of complexity.
Just my two cents. I might be totally off here of course.
Christian
next prev parent reply other threads:[~2017-11-08 19:02 UTC|newest]
Thread overview: 29+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-11-03 0:44 [PATCH resend 2/2] userns: control capabilities of some user namespaces Mahesh Bandewar
[not found] ` <20171103004436.40026-1-mahesh-bmGAjcP2qsnk1uMJSBkQmQ@public.gmane.org>
2017-11-04 23:53 ` Serge E. Hallyn
[not found] ` <20171104235346.GA17170-7LNsyQBKDXoIagZqoN9o3w@public.gmane.org>
2017-11-06 7:23 ` Mahesh Bandewar (महेश बंडेवार)
2017-11-06 15:03 ` Serge E. Hallyn
[not found] ` <20171106150302.GA26634-7LNsyQBKDXoIagZqoN9o3w@public.gmane.org>
2017-11-06 21:33 ` [kernel-hardening] " Daniel Micay
[not found] ` <1510003994.736.0.camel-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
2017-11-06 22:14 ` Serge E. Hallyn
[not found] ` <20171106221418.GA32543-7LNsyQBKDXoIagZqoN9o3w@public.gmane.org>
2017-11-06 22:42 ` Christian Brauner
2017-11-07 2:16 ` Daniel Micay
[not found] ` <1510020963.736.42.camel-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
2017-11-07 3:23 ` Serge E. Hallyn
2017-11-09 18:01 ` chris hyser
[not found] ` <da764cbf-7522-06a0-6c21-adfa3eaac9c2-QHcLZuEGTsvQT0dZR+AlfA@public.gmane.org>
2017-11-09 18:05 ` Serge E. Hallyn
2017-11-09 18:27 ` chris hyser
2017-11-06 23:17 ` Boris Lukashev
2017-11-06 23:39 ` Serge E. Hallyn
2017-11-07 0:01 ` Boris Lukashev
[not found] ` <CAFUG7CcW077LHcQEqk7qy7iVvmi-3J8psD1Kwj45XvHThiZC6w-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2017-11-07 3:28 ` [kernel-hardening] " Serge E. Hallyn
[not found] ` <20171107032802.GA6669-7LNsyQBKDXoIagZqoN9o3w@public.gmane.org>
2017-11-08 11:09 ` Mahesh Bandewar (महेश बंडेवार)
2017-11-08 19:02 ` Christian Brauner [this message]
[not found] ` <20171108190223.vdkyepcaegmub6le-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
2017-11-09 0:55 ` Mahesh Bandewar (महेश बंडेवार)
[not found] ` <CAF2d9jjed4Q7QvCD9Kpaa7L-Ngg3XFbJvt0jNVUUwt=52wDjjw-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2017-11-09 3:21 ` Serge E. Hallyn
2017-11-09 7:13 ` Mahesh Bandewar (महेश बंडेवार)
2017-11-09 7:18 ` Mahesh Bandewar (महेश बंडेवार)
2017-11-09 16:14 ` Serge E. Hallyn
[not found] ` <CAF2d9jgs5MYn1dMT2mbhF=6UB2Hoo5kwmJhXuE6memBfWzkWXQ-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2017-11-09 21:58 ` [kernel-hardening] " Eric W. Biederman
[not found] ` <871sl7dsh8.fsf-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org>
2017-11-10 4:30 ` Mahesh Bandewar (महेश बंडेवार)
2017-11-10 4:46 ` Serge E. Hallyn
[not found] ` <20171110044645.GA3694-7LNsyQBKDXoIagZqoN9o3w@public.gmane.org>
2017-11-10 5:28 ` Mahesh Bandewar (महेश बंडेवार)
2017-11-09 17:25 ` Serge E. Hallyn
2017-11-10 1:49 ` Mahesh Bandewar (महेश बंडेवार)
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