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

[Josh Boyer]

On Tue, Jan 26, 2016 at 9:46 AM, Austin S. Hemmelgarn
<ahferroin7@xxxxxxxxx> wrote:
> On 2016-01-26 09:38, Josh Boyer wrote:
>>
>> On Mon, Jan 25, 2016 at 11:57 PM, Eric W. Biederman
>> <ebiederm@xxxxxxxxxxxx> wrote:
>>>
>>> Kees Cook <keescook@xxxxxxxxxxxx> writes:
>>>
>>>> On Mon, Jan 25, 2016 at 11:33 AM, Eric W. Biederman
>>>> <ebiederm@xxxxxxxxxxxx> wrote:
>>>>>
>>>>> Kees Cook <keescook@xxxxxxxxxxxx> 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@xxxxxxxxxxxx>
>>>>>
>>>>> 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
>> distro kernel as-is, which may not meet their goal of having userns
>> disabled, or 2) rebuild their kernel to disable it, which may
>> invalidate any support contracts they have.
>>
>> I tend to agree with you on the lack of value around runtime
>> mitigation, but allowing an admin to toggle this as a blatant on/off
>> switch on reboot does have value.
>>
>>>> This feature is already implemented by two distros, and likely wanted
>>>> by others. We cannot ignore that. The sysctl default doesn't change
>>>> the existing behavior, so this doesn't get in your way at all. Can you
>>>> please respond to my earlier email where I rebutted each of your
>>>> arguments against it? Just saying "no" and putting words in my mouth
>>>> isn't very productive.
>>>
>>>
>>> Calling people who make mistakes insane is not a rebuttal.  In security
>>> usability matters, and your sysctl has low usability.
>>>
>>> Further you seem to have missed something crucial in your understanding.
>>> As was explained earlier the sysctl was added to ubuntu to allow early
>>> adopters to experiment not as a long term way of managing user
>>> namespaces.
>>>
>>>
>>> What sounds like a generally useful feature that would cover your use
>>> case and many others is a per user limit on the number of user
>>> namespaces users may create.
>>
>>
>> Where that number may be zero?  I don't see how that is really any
>> better than a sysctl.  Could you elaborate?
>
> It's a better option because it would allow better configurability. Take for
> example a single user desktop system with some network daemons.  On such a
> system, the actual login used for the graphical environment by the user
> should be allowed at least a few user namespaces, because some software
> depends on them for security (Chrome for example, as well as some distro's
> build systems), but system users should be limited to at most one if they
> need it, and ideally zero, so that remote exploits couldn't give access to a
> user namespace.
>
> Conversely, on a server system, it's not unreasonable to completely disable
> user namespaces for almost everything, except for giving one to services
> that use them properly for sand-boxing.

OK, so better granularity.  Fine.

> I will state though that I only feel this is a better solution given that
> two criteria are met:
> 1. You can set 0 as the limit.
> 2. You can configure this without needing some special software (this in
> particular means that seccomp is not an option).

I'd have to add 3. You can set a global default for all users that can
be overridden on a per user basis.

Otherwise you play whack-a-mole with every new user or daemon that
adds its own uid.

josh