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From:   Song Liu <songliubraving@fb.com>
To:     "Serge E. Hallyn" <serge@hallyn.com>
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Subject: Re: [PATCH v5 0/4] Introduce security_create_user_ns()
Thread-Topic: [PATCH v5 0/4] Introduce security_create_user_ns()
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Date:   Fri, 26 Aug 2022 17:00:51 +0000
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> 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.

Song

> 
>> Failing create_user_ns() doesn't make the system more reliable. 
>> Specifically, we call create_user_ns() via two paths: fork/clone and 
>> unshare. For both paths, we need the userspace to use user_namespace, 
>> and to honor failed create_user_ns(). 
>> 
>> On the other hand, I would echo that killing the process is not 
>> practical in some use cases. Specifically, allowing the application to 
>> run in a less secure environment for a short period of time might be 
>> much better than killing it and taking down the whole service. Of 
>> course, there are other cases that security is more important, and 
>> taking down the whole service is the better choice. 
>> 
>> I guess the ultimate solution is a way to enforce using user_namespace
>> in the kernel (if it ever makes sense...). But I don't know how that
>> gonna work. Before we have such solution, maybe we only need an 
>> void hook for observability (or just a tracepoint, coming from BPF
>> background). 
>> 
>> Thanks,
>> Song