pyxis: allow unprivileged user namespaces so enroot works on Ubuntu 23.10+

[100milliongold]

Problem

Ubuntu 23.10 and later set kernel.apparmor_restrict_unprivileged_userns=1 by default. Under that default, enroot-nsenter fails as soon as pyxis starts a container:

enroot-nsenter: failed to create user namespace: Permission denied

The nvidia.enroot galaxy role does not handle this, and DeepOps lists Ubuntu 24.04 LTS as a supported OS, so container jobs do not work out of the box on a supported platform.

Observed on DGX OS 7.5.0 (Ubuntu-based), Slurm 26.05.1, pyxis 0.11.1.

Fix

Add a sysctl task in roles/pyxis for compute nodes. The task is gated on the presence of /proc/sys/kernel/apparmor_restrict_unprivileged_userns rather than on ansible_distribution_version, so it is a no-op on kernels built without AppArmor userns restrictions and does not need updating for future releases.

Note on hardening

This relaxes a system-wide hardening default: unprivileged user namespaces are what AppArmor is restricting here. A narrower alternative is an AppArmor profile scoped to enroot-nsenter (as packaged by some distributions for other userns consumers). If maintainers prefer that approach, I am happy to rework the patch.

The sysctl is written to /etc/sysctl.d/60-enroot-userns.conf so it is visible and revertible, rather than being applied only at runtime.

[dholt]

Please replace the unconditional host-wide sysctl change with a narrowly scoped AppArmor policy for enroot-nsenter and provide evidence from a real Pyxis/Enroot container launch on an affected supported OS. If a global sysctl fallback is still needed for unusual installations, make it an explicit, documented, default-off administrator choice rather than changing the security default whenever the kernel knob exists.

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Automated triage review (agent-generated on the maintainer's behalf; a human maintainer decides merges).