Re: [PATCH v4 0/4] Introduce security_create_user_ns()

Casey Schaufler
2022-08-09T17:43:51Z

On 8/9/2022 9:07 AM, Eric W. Biederman wrote:
> Paul Moore <paul@paul-moore.com> writes:
>
>> On Mon, Aug 8, 2022 at 3:43 PM Eric W. Biederman <ebiederm@xmission.com> wrote:
>>> "Eric W. Biederman" <ebiederm@xmission.com> writes:
>>>> Paul Moore <paul@paul-moore.com> writes:
>>>>
>>>>>> I did provide constructive feedback. My feedback to his problem
>>>>>> was to address the real problem of bugs in the kernel.
>>>>> We've heard from several people who have use cases which require
>>>>> adding LSM-level access controls and observability to user namespace
>>>>> creation. This is the problem we are trying to solve here; if you do
>>>>> not like the approach proposed in this patchset please suggest another
>>>>> implementation that allows LSMs visibility into user namespace
>>>>> creation.
>>>> Please stop, ignoring my feedback, not detailing what problem or
>>>> problems you are actually trying to be solved, and threatening to merge
>>>> code into files that I maintain that has the express purpose of breaking
>>>> my users.
>>>>
>>>> You just artificially constrained the problems, so that no other
>>>> solution is acceptable. On that basis alone I am object to this whole
>>>> approach to steam roll over me and my code.
>>> If you want an example of what kind of harm it can cause to introduce a
>>> failure where no failure was before I invite you to look at what
>>> happened with sendmail when setuid was modified to fail, when changing
>>> the user of a process would cause RLIMIT_NPROC to be exceeded.
>> I think we are all familiar with the sendmail capabilities bug and the
>> others like it, but using that as an excuse to block additional access
>> controls seems very weak. The Linux Kernel is very different from
>> when the sendmail bug hit (what was that, ~20 years ago?), with
>> advancements in capabilities and other discretionary controls, as well
>> as mandatory access controls which have enabled Linux to be certified
>> through a number of third party security evaluations.
> If you are familiar with scenarios like that then why is there not
> being due diligence performed to ensure that userspace won't break?
>
> Certainly none of the paperwork you are talking about does that kind
> of checking and it most definitely is not happening before the code
> gets merged.
>
> I am saying that performing that due diligence should be a requirement
> before anyone even thinks about merging a patch that adds permission
> checks where no existed before.
>
> Sometimes changes to fix security bugs can get away with adding new
> restrictions because we know with a very very high degree of probability
> that the only thing that will break will be exploit code. In the rare
> case when real world applications are broken such changes need to be
> reverted or adapted. No one has even made the argument that only
> exploit code will be affected.
>
> So I am sorry I am the one who has to be the one to get in the way of a
> broken process with semantic review, but due diligence has not been
> done. So I am say no way this code should be merged.
>
>
> In addition to actually breaking existing userspace, I think there is a
> very real danger of breaking userspace, I think there is a very real
> danger of breaking network effects by making such a large change to the
> design of user namespaces.
>
>
>>> I am not arguing that what you are proposing is that bad but unexpected
>>> failures cause real problems, and at a minimum that needs a better
>>> response than: "There is at least one user that wants a failure here".
>> Let me fix that for you: "There are multiple users who want to have
>> better visibility and access control for user namespace creation."
> Visibility sure. Design a proper hook for that. All the proposed hook
> can do is print an audit message. It can't allocate or manage any state
> as there is not the corresponding hook when a user namespace is freed.
> So the proposed hook is not appropriate for increasing visibility.
>
>
> Access control. Not a chance unless it is carefully designed and
> reviewed. There is a very large cost to adding access control where
> it has not previously existed.
>
> I talk about that cost as people breaking my users as that is how I see
> it. I don't see any discussion on why I am wrong.
>
> If we are going to add an access controls I want to see someone point
> out something that is actually semantically a problem. What motivates
> an access control?

Smack has no interest in using the proposed hook because user namespaces
are neither subjects nor objects. They are collections of DAC and/or
privilege configuration alternatives. Or something like that. From the
viewpoint of a security module that only implements old fashioned MAC
there is no value in constraining user namespaces.

SELinux, on the other hand, seeks to be comprehensive well beyond
controlling accesses between subjects and objects. Asking the question
"should there be a control on this operation?" seems sufficient to justify
adding the control to SELinux policy. This is characteristic of
"Fine Grain" control.

So I'm of two minds on this. I don't need the hook, but I also understand
why SELinux and BPF want it. I don't necessarily agree with their logic,
but it is consistent with existing behavior. Any system that uses either
of those security modules needs to be ready for unexpected denials based
on any potential security concern. Keeping namespaces completely orthogonal
to LSM seems doomed to failure eventually.

>
> So far the only answer I have received is people want to reduce the
> attack surface of the kernel. I don't possibly see how reducing the
> attack surface by removing user namespaces makes the probability of
> having an exploitable kernel bug, anything approaching zero.
>
> So I look at the calculus. Chance of actually breaking userspace, or
> preventing people with a legitimate use from using user namespaces > 0%.
> Chance of actually preventing a determined attacker from exploiting the
> kernel < 1%. Amount of work to maintain, non-zero, and I really don't
> like it.
>
> Lots of work to achieve nothing but breaking some of my users.
>
> So please stop trying to redesign my subsystem and cause me headaches,
> unless you are going to do the due diligence necessary to do so
> responsibly.
>
> Eric