Re: [PATCH v3] security: Expand task_setscheduler LSM hook to include CPU affinity mask

[Peter Zijlstra]

On Wed, May 27, 2026 at 01:41:52PM -0400, Aaron Tomlin wrote:

> > > The actual use case here is multi-tenant workload isolation and visibility.
> > > Passing the evaluated cpumask to the BPF LSM allows operators to write a
> > > simple eBPF program to detect spatial boundary overlaps (e.g., logging an
> > > event if a requested mask intersects with platform-reserved cores).

Why isn't cgroups good enough to enforce this? If you create a cgroup
hierarchy per tenant, and constrain them using the cpuset controller,
they should not be able to escape, rendering this event impossible.

> > > If this justification makes more sense, I will focus strictly on the
> > > seccomp pointer limitations and multi-tenant workload isolation.
> > 
> > I suppose it does, my only remaining question is if that is indeed
> > proper use of LSM -- I really don't know much about that.
> > 
> 
> We are not creating a bespoke BPF hook here; rather, we are rectifying a
> historical blind spot within the API. The existing LSM hook is invoked
> during sched_setaffinity(), yet it presently receives only the task_struct
> pointer. Consequently, the security module is essentially asked, "Should
> Process A be permitted to alter Process B's affinity?" without being
> informed of the proposed affinity itself. Providing in_mask simply
> furnishes the existing hook with the requisite payload to make an informed
> decision.

It occurs to me that this same argument would require to also pass in
the new sched_attr, no? That way the LSM can inspect the new policy
before it becomes effective.

> Were the objective solely one of observability, a tracepoint would indeed
> be the most suitable mechanism. However, if the aim within multi-tenant
> environments is active enforcement (namely, safely returning -EPERM to deny
> the pinning request before the scheduler applies it), the LSM layer remains
> the standard, architecturally supported gateway for returning syscall
> errors in accordance with administrative policy.

Indeed; but being constrained in a cpuset cgroup would result in the
same, no?

> I shall defer to Paul Moore and the LSM maintainers for their final
> blessing on the LSM API semantics.

Yes, I think that this is an interesting test-case of the LSM purpose.

You seem to be mostly aiming at resource control, something that is
traditionally done elsewhere.

> Thank you once again for the thorough review and for keeping the
> architectural boundaries honest.

No problem, just trying to understand myself ;-)

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