Re: [kernel-hardening] Re: [PATCH 0/2] sysctl: allow CLONE_NEWUSER to be disabled

[Andy Lutomirski <luto@amacapital.net>]

On Tue, Jan 26, 2016 at 10:09 AM, Austin S. Hemmelgarn
<ahferroin7@gmail.com> wrote:
> On 2016-01-26 12:15, Serge Hallyn wrote:
>>
>> Quoting Josh Boyer (jwboyer@fedoraproject.org):
>>>
>>> On Mon, Jan 25, 2016 at 11:57 PM, Eric W. Biederman
>>> <ebiederm@xmission.com> wrote:
>>>>
>>>> Kees Cook <keescook@chromium.org> writes:
>>>>
>>>>> On Mon, Jan 25, 2016 at 11:33 AM, Eric W. Biederman
>>>>> <ebiederm@xmission.com> wrote:
>>>>>>
>>>>>> Kees Cook <keescook@chromium.org> writes:
>>>>>>>
>>>>>>>
>>>>>>> Well, I don't know about less weird, but it would leave a unneeded
>>>>>>> hole in the permission checks.
>>>>>>
>>>>>>
>>>>>> To be clear the current patch has my:
>>>>>>
>>>>>> Nacked-by: "Eric W. Biederman" <ebiederm@xmission.com>
>>>>>>
>>>>>> The code is buggy, and poorly thought through.  Your lack of interest
>>>>>> in
>>>>>> fixing the bugs in your patch is distressing.
>>>>>
>>>>>
>>>>> I'm not sure where you see me having a "lack of interest". The
>>>>> existing cap-checking sysctls have a corner-case bug, which is
>>>>> orthogonal to this change.
>>>>
>>>>
>>>> That certainly doesn't sound like you have any plans to change anything
>>>> there.
>>>>
>>>>>> So broken code, not willing to fix.  No. We are not merging this
>>>>>> sysctl.
>>>>>
>>>>>
>>>>> I think you're jumping to conclusions. :)
>>>>
>>>>
>>>> I think I am the maintainer.
>>>>
>>>> What you are proposing is very much something that is only of interst to
>>>> people who are not using user namespaces.  It is fatally flawed as
>>>> a way to avoid new attack surfaces for people who don't care as the
>>>> sysctl leaves user namespaces enabled by default.  It is fatally flawed
>>>> as remediation to recommend to people to change if a new user namespace
>>>> related but is discovered.  Any running process that happens to be
>>>> created while user namespace creation was enabled will continue to
>>>> exist.  Effectively a reboot will be required as part of a mitigation.
>>>> Many sysadmins will get that wrong.
>>>>
>>>> I can't possibly see your sysctl as proposed achieving it's goals.  A
>>>> person has to be entirely too aware of subtlety and nuance to use it
>>>> effectively.
>>>
>>>
>>> What you're saying is true for the "oh crap" case of a new userns
>>> related CVE being found.  However, there is the case where sysadmins
>>> know for a fact that a set of machines should not allow user
>>> namespaces to be enabled.  Currently they have 2 choices, 1) use their
>>
>>
>> Hi - can you give a specific example of this?  (Where users really should
>> not be able to use them - not where they might not need them)  I think
>> it'll help the discussion tremendously.  Because so far the only good
>> arguments I've seen have been about actual bugs in the user namespaces,
>> which would not warrant a designed-in permanent disable switch.  If
>> there are good use cases where such a disable switch will always be
>> needed (and compiling out can't satisfy) that'd be helpful.
>
> In general, if a particular daemon provides a network service and does not
> use user namespaces for sand-boxing, it should not be allowed to use user
> namespaces, because those then become something else to potentially land an
> exploit through.  ntpd, postfix, and most other regularly used network
> servers fall into this category.

seccomp handles this issue quite nicely.

>
> If you're hosting a shared system providing terminal server like usage where
> the users actually have shell access, then they probably should not be able
> to use user namespaces on the server.
>

Au contraire.  If they have user ns access, then can sandbox their own programs.

--Andy