Re: [lxc-devel] [RFC PATCH 00/11] Add support for devtmpfs in user namespaces

[Marian Marinov]

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On 05/20/2014 05:19 PM, Serge Hallyn wrote:
> Quoting Andy Lutomirski (luto@xxxxxxxxxxxxxx):
>> On May 15, 2014 1:26 PM, "Serge E. Hallyn" <serge@xxxxxxxxxx> wrote:
>>> 
>>> Quoting Richard Weinberger (richard@xxxxxx):
>>>> Am 15.05.2014 21:50, schrieb Serge Hallyn:
>>>>> Quoting Richard Weinberger (richard.weinberger@xxxxxxxxx):
>>>>>> On Thu, May 15, 2014 at 4:08 PM, Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> 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.
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