LSM namespacing API

[Dr. Greg]

On Thu, Aug 21, 2025 at 07:57:11AM -0700, John Johansen wrote:

Good morning, I hope the week is starting well for everyone.

Now that everyone is getting past the summer holiday season, it would
seem useful to specifically clarify some of the LSM namespace
implementation details.

> On 8/21/25 07:26, Serge E. Hallyn wrote:
> >On Thu, Aug 21, 2025 at 12:46:10AM -0700, John Johansen wrote:
> >>On 8/19/25 10:47, Stephen Smalley wrote:
> >>>On Tue, Aug 19, 2025 at 10:56???AM Paul Moore <paul at paul-moore.com> 
> >>>wrote:
> >>>>
> >>>>Hello all,
> >>>>
> >>>>As most of you are likely aware, Stephen Smalley has been working on
> >>>>adding namespace support to SELinux, and the work has now progressed
> >>>>to the point where a serious discussion on the API is warranted.  For
> >>>>those of you are unfamiliar with the details or Stephen's patchset, or
> >>>>simply need a refresher, he has some excellent documentation in his
> >>>>work-in-progress repo:
> >>>>
> >>>>* https://github.com/stephensmalley/selinuxns
> >>>>
> >>>>Stephen also gave a (pre-recorded) presentation at LSS-NA this year
> >>>>about SELinux namespacing, you can watch the presentation here:
> >>>>
> >>>>* https://www.youtube.com/watch?v=AwzGCOwxLoM
> >>>>
> >>>>In the past you've heard me state, rather firmly at times, that I
> >>>>believe namespacing at the LSM framework layer to be a mistake,
> >>>>although if there is something that can be done to help facilitate the
> >>>>namespacing of individual LSMs at the framework layer, I would be
> >>>>supportive of that.  I think that a single LSM namespace API, similar
> >>>>to our recently added LSM syscalls, may be such a thing, so I'd like
> >>>>us to have a discussion to see if we all agree on that, and if so,
> >>>>what such an API might look like.
> >>>>
> >>>>At LSS-NA this year, John Johansen and I had a brief discussion where
> >>>>he suggested a single LSM wide clone*(2) flag that individual LSM's
> >>>>could opt into via callbacks.  John is directly CC'd on this mail, so
> >>>>I'll let him expand on this idea.
> >>>>
> >>>>While I agree with John that a fs based API is problematic (see all of
> >>>>our discussions around the LSM syscalls), I'm concerned that a single
> >>>>clone*(2) flag will significantly limit our flexibility around how
> >>>>individual LSMs are namespaced, something I don't want to see happen.
> >>>>This makes me wonder about the potential for expanding
> >>>>lsm_set_self_attr(2) to support a new LSM attribute that would support
> >>>>a namespace "unshare" operation, e.g. LSM_ATTR_UNSHARE.  This would
> >>>>provide a single LSM framework API for an unshare operation while also
> >>>>providing a mechanism to pass LSM specific via the lsm_ctx struct if
> >>>>needed.  Just as we do with the other LSM_ATTR_* flags today,
> >>>>individual LSMs can opt-in to the API fairly easily by providing a
> >>>>setselfattr() LSM callback.
> >>>>
> >>>>Thoughts?
> >>>
> >>>I think we want to be able to unshare a specific security module
> >>>namespace without unsharing the others, i.e. just SELinux or just
> >>>AppArmor.
> >>
> >>yes which is part of the problem with the single flag. That choice
> >>would be entirely at the policy level, without any input from userspace.
> >
> >AIUI Paul's suggestion is the user can pre-set the details of which
> >lsms to unshare and how with the lsm_set_self_attr(), and then a
> >single CLONE_LSM effects that.

> yes, I was specifically addressing the conversation I had with Paul at
> LSS that Paul brought up. That is
> 
>   At LSS-NA this year, John Johansen and I had a brief discussion where
>   he suggested a single LSM wide clone*(2) flag that individual LSM's
>   could opt into via callbacks.
> 
> the idea there isn't all that different than what Paul proposed. You
> could have a single flag, if you can provide ancillary information. But
> a single flag on its own isn't sufficient.

If one thing has come out of this thread, it would seem to be the fact
that there is going to be little commonality in the requirements that
various LSM's will have for the creation of a namespace.

Given that, the most infrastructure that the LSM should provide would
be a common API for a resource orchestrator to request namespace
separation and to provide a framework for configuring the namespace
prior to when execution begins in the context of the namespace.

The first issue to resolve would seem to be what namespace separation
implies.

John, if I interpret your comments in this discussion correctly, your
contention is that when namespace separation is requested, all of the
LSM's that implement namespaces will create a subordinate namespace,
is that a correct assumption?

It would seem, consistent with the 'stacking' concept, that any LSM
with namespace capability that chooses not to separate, will result in
denial of the separation request.  That in turn will imply the need to
unwind or delete any namespace context that other LSM's may have
allocated before the refusal occurred.

This model also implies that the orchestrator requesting the
separation will need to pass a set of parameters describing the
characteristics of each namespace, described by the LSM identifier
that they pertain to.  Since there may be a need to configure multiple
namespaces there would be a requirement to pass an array or list of
these parameter sets.

There will also be a need to inject, possibly substantial amounts of
policy or model information into the namespace, before execution in
the context of the namespace begins.

There will also be a need to decide whether namespace separation
should occur at the request of the orchestrator or at the next fork,
the latter model being what the other resource namespaces use.  We
believe the argument for direct separation can be made by looking at
the gymnastics that orchestrators need to jump through with the
'change-on-fork' model.

Case in point, it would seem realistic that a process with sufficient
privilege, may desire to place itself in a new LSM namespace context
in a manner that does not require re-execution of itself.

With respect to separation, the remaining issue is if a new security
capability bit needs to be implemented to gate namespace separation.
John, based on your comments, I believe you would support this need?

> You can do a subset with a single flag and only policy directing things,
> but that would cut container managers out of the decision. Without a
> universal container identifier that really limits what you can do. In
> another email I likend it to the MCS label approach to the container
> where you have a single security policy for the container and each
> container gets to be a unique instance of that policy. Its not a perfect
> analogy as with namespace policy can be loaded into the namespace making
> it unique. I don't think the approach is right because not all namespaces
> implement a loadable policy, and even when they do I think we can do a
> better job if the container manager is allowed to provide additional
> context with the namespacing request.

In order to be relevant, the configuration of LSM namespaces need to
be under control of a resource orchestrator or container manager.

What we hear from people doing Kubernetes, at scale, is a desire to be
able to request that a container be run somewhere in the hardware
resource pool and for that container to implement a security model
specific to the needs of the workload running in that container.  In a
manner that is orthogonal from other security policies that may be in
effect for other workloads, on the host or in other containers.

Hopefully the above will be of assistance in furthering discussion.

Have a good week.

As always,
Dr. Greg

The Quixote Project - Flailing at the Travails of Cybersecurity
              https://github.com/Quixote-Project