Re: [PATCH v5 0/4] Introduce security_create_user_ns()

[Song Liu <songliubraving@fb.com>]

> On Aug 26, 2022, at 2:00 PM, Serge E. Hallyn <serge@hallyn.com> wrote:
> 
> On Fri, Aug 26, 2022 at 05:00:51PM +0000, Song Liu wrote:
>> 
>> 
>>> On Aug 26, 2022, at 8:24 AM, Serge E. Hallyn <serge@hallyn.com> wrote:
>>> 
>>> On Thu, Aug 25, 2022 at 09:58:46PM +0000, Song Liu wrote:
>>>> 
>>>> 
>>>>> On Aug 25, 2022, at 12:19 PM, Paul Moore <paul@paul-moore.com> wrote:
>>>>> 
>>>>> On Thu, Aug 25, 2022 at 2:15 PM Eric W. Biederman <ebiederm@xmission.com> wrote:
>>>>>> Paul Moore <paul@paul-moore.com> writes:
>>>>>>> On Fri, Aug 19, 2022 at 10:45 AM Serge E. Hallyn <serge@hallyn.com> wrote:
>>>>>>>> I am hoping we can come up with
>>>>>>>> "something better" to address people's needs, make everyone happy, and
>>>>>>>> bring forth world peace.  Which would stack just fine with what's here
>>>>>>>> for defense in depth.
>>>>>>>> 
>>>>>>>> You may well not be interested in further work, and that's fine.  I need
>>>>>>>> to set aside a few days to think on this.
>>>>>>> 
>>>>>>> I'm happy to continue the discussion as long as it's constructive; I
>>>>>>> think we all are.  My gut feeling is that Frederick's approach falls
>>>>>>> closest to the sweet spot of "workable without being overly offensive"
>>>>>>> (*cough*), but if you've got an additional approach in mind, or an
>>>>>>> alternative approach that solves the same use case problems, I think
>>>>>>> we'd all love to hear about it.
>>>>>> 
>>>>>> I would love to actually hear the problems people are trying to solve so
>>>>>> that we can have a sensible conversation about the trade offs.
>>>>> 
>>>>> Here are several taken from the previous threads, it's surely not a
>>>>> complete list, but it should give you a good idea:
>>>>> 
>>>>> https://lore.kernel.org/linux-security-module/CAHC9VhQnPAsmjmKo-e84XDJ1wmaOFkTKPjjztsOa9Yrq+AeAQA@mail.gmail.com/
>>>>> 
>>>>>> As best I can tell without more information people want to use
>>>>>> the creation of a user namespace as a signal that the code is
>>>>>> attempting an exploit.
>>>>> 
>>>>> Some use cases are like that, there are several other use cases that
>>>>> go beyond this; see all of our previous discussions on this
>>>>> topic/patchset.  As has been mentioned before, there are use cases
>>>>> that require improved observability, access control, or both.
>>>>> 
>>>>>> As such let me propose instead of returning an error code which will let
>>>>>> the exploit continue, have the security hook return a bool.  With true
>>>>>> meaning the code can continue and on false it will trigger using SIGSYS
>>>>>> to terminate the program like seccomp does.
>>>>> 
>>>>> Having the kernel forcibly exit the process isn't something that most
>>>>> LSMs would likely want.  I suppose we could modify the hook/caller so
>>>>> that *if* an LSM wanted to return SIGSYS the system would kill the
>>>>> process, but I would want that to be something in addition to
>>>>> returning an error code like LSMs normally do (e.g. EACCES).
>>>> 
>>>> I am new to user_namespace and security work, so please pardon me if
>>>> anything below is very wrong. 
>>>> 
>>>> IIUC, user_namespace is a tool that enables trusted userspace code to 
>>>> control the behavior of untrusted (or less trusted) userspace code. 
>>> 
>>> No.  user namespaces are not a way for more trusted code to control the
>>> behavior of less trusted code.
>> 
>> Hmm.. In this case, I think I really need to learn more. 
>> 
>> Thanks for pointing out my misunderstanding.
> 
> (I thought maybe Eric would chime in with a better explanation, but I'll
> fill it in for now :)
> 
> One of the main goals of user namespaces is to allow unprivileged users
> to do things like chroot and mount, which are very useful development
> tools, without needing admin privileges.  So it's almost the opposite
> of what you said: rather than to enable trusted userspace code to control
> the behavior of less trusted code, it's to allow less privileged code to
> do things which do not affect other users, without having to assume *more*
> privilege.

Thanks for the explanation! 

> 
> To be precise, the goals were:
> 
> 1. uid mapping - allow two users to both "use uid 500" without conflicting
> 2. provide (unprivileged) users privilege over their own resources
> 3. absolutely no extra privilege over other resources
> 4. be able to nest

Now I have better idea about "what". But I am not quite sure about how to do
it. I will do more homework, and probably come back with more questions. :)

> 
> While (3) was technically achieved, the problem we have is that
> (2) provides unprivileged users the ability to exercise kernel code
> which they previously could not.

Do you mean this one?

"""
I think the problem is that it seems
you can pretty reliably get a root shell at some point in the future
by creating a user namespace, leaving it open for a bit, and waiting
for a new announcement of the latest netfilter or whatever exploit
that requires root in a user namespace.  Then go back to your userns
shell and run the exploit.
"""

Please don't share how to do it yet. I want to use it as a test for my study. :)

Thanks again!

Song