Seccomp on Ethan-ZYF

Week2 Day 4: K8s security — AppArmor

Thu, 21 May 2026 00:00:00 -0700

AppArmor as the path-based complement to seccomp’s syscall-based filtering — and a hands-on lesson in why that complement is harder to deploy. This post covers how K8s wraps AppArmor (securityContext.appArmorProfile, symmetric with seccomp’s three types), why an AppArmor profile is heavier than a seccomp one (it must be pre-loaded into the node’s kernel via apparmor_parser, not just dropped as a file), why deny /data/** w blocks every syscall that writes there (closing the hole where touch bypassed yesterday’s mkdir block), and the day’s most valuable lesson: the experiment couldn’t run at all because a Mac/Docker-Desktop kind node has no AppArmor LSM in its kernel — a firsthand encounter with the fail-open trap and the portability problem of AppArmor-based mitigations.

Week2 Day 3: K8s security — seccomp

Wed, 20 May 2026 00:00:00 -0700

Where Week 1 and Week 2 meet: K8s seccomp is the same kernel seccomp from Week 1, except runc installs the BPF filter for you instead of your application. This post traces the full kubelet → containerd → runc chain (and what runc actually does — namespaces, cgroups, NNP, caps, seccomp, AppArmor, then exec), the three profile types (Unconfined / RuntimeDefault / Localhost), why the profile JSON is just the declarative form of the cBPF you’d hand-write, why SCMP_ACT_ERRNO lets the process survive while mkdir returns EPERM, and the crucial limitation: seccomp blocks syscalls, not intent — block mkdir and touch (which uses openat) still creates files. Plus /proc/<pid>/status as the ground-truth proof a filter is loaded.

Day 4: seccomp BPF Filter (filter mode deep dive)

Thu, 14 May 2026 00:00:00 -0700

A deep dive into seccomp filter mode: the BPF data layout, why pointer args can’t be dereferenced (TOCTOU and atomic context), the cBPF instruction skeleton, the multi-ABI bypasses every filter must defend against, and the eight SECCOMP_RET_* actions that power modern container security — including the USER_NOTIF + fd-injection pattern that runc/crun use.

Day 3: seccomp Basics + Strict Mode

Wed, 13 May 2026 00:00:00 -0700

How seccomp intercepts syscalls at the kernel entry, what SECCOMP_MODE_STRICT actually allows (and the _exit() trap that fools nearly everyone), plus the design philosophy that filter mode inherits: monotonic, irrevocable security state.