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SubjectRe: [lxc-devel] [RFC PATCH 00/11] Add support for devtmpfs in user namespaces
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On 05/20/2014 05:19 PM, Serge Hallyn wrote:
> Quoting Andy Lutomirski (luto@amacapital.net):
>> On May 15, 2014 1:26 PM, "Serge E. Hallyn" <serge@hallyn.com> wrote:
>>>
>>> Quoting Richard Weinberger (richard@nod.at):
>>>> Am 15.05.2014 21:50, schrieb Serge Hallyn:
>>>>> Quoting Richard Weinberger (richard.weinberger@gmail.com):
>>>>>> On Thu, May 15, 2014 at 4:08 PM, Greg Kroah-Hartman <gregkh@linuxfoundation.org> wrote:
>>>>>>> Then don't use a container to build such a thing, or fix the build scripts to not do that :)
>>>>>>
>>>>>> I second this. To me it looks like some folks try to (ab)use Linux containers for purposes where KVM
>>>>>> would much better fit in. Please don't put more complexity into containers. They are already horrible
>>>>>> complex and error prone.
>>>>>
>>>>> I, naturally, disagree :) The only use case which is inherently not valid for containers is running a
>>>>> kernel. Practically speaking there are other things which likely will never be possible, but if someone
>>>>> offers a way to do something in containers, "you can't do that in containers" is not an apropos response.
>>>>>
>>>>> "That abstraction is wrong" is certainly valid, as when vpids were originally proposed and rejected,
>>>>> resulting in the development of pid namespaces. "We have to work out (x) first" can be valid (and I can
>>>>> think of examples here), assuming it's not just trying to hide behind a catch-22/chicken-egg problem.
>>>>>
>>>>> Finally, saying "containers are complex and error prone" is conflating several large suites of userspace
>>>>> code and many kernel features which support them. Being more precise would, if the argument is valid, lend
>>>>> it a lot more weight.
>>>>
>>>> We (my company) use Linux containers since 2011 in production. First LXC, now libvirt-lxc. To understand the
>>>> internals better I also wrote my own userspace to create/start containers. There are so many things which can
>>>> hurt you badly. With user namespaces we expose a really big attack surface to regular users. I.e. Suddenly a
>>>> user is allowed to mount filesystems.
>>>
>>> That is currently not the case. They can mount some virtual filesystems and do bind mounts, but cannot mount
>>> most real filesystems. This keeps us protected (for now) from potentially unsafe superblock readers in the
>>> kernel.
>>>
>>>> Ask Andy, he found already lots of nasty things...
>>
>> I don't think I have anything brilliant to add to this discussion right now, except possibly:
>>
>> ISTM that Linux distributions are, in general, vulnerable to all kinds of shenanigans that would happen if an
>> untrusted user can cause a block device to appear. That user doesn't need permission to mount it
>
> Interesting point. This would further suggest that we absolutely must ensure that a loop device which shows up in
> the container does not also show up in the host.

Can I suggest the usage of the devices cgroup to achieve that?

Marian

>
>> or even necessarily to change its contents on the fly.
>>
>> E.g. what happens if you boot a machine that contains a malicious disk image that has the same partition UUID as
>> /? Nothing good, I imagine.
>>
>> So if we're going to go down this road, we really need some way to tell the host that certain devices are not
>> trusted.
>>
>> --Andy
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- --
Marian Marinov
Founder & CEO of 1H Ltd.
Jabber/GTalk: hackman@jabber.org
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Mobile: +359 886 660 270
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