diskless-workstation/docs/stig-deviations.md

# STIG deviation register — diskless fleet image

DISA STIG (SSG `xccdf_org.ssgproject.content_profile_stig` for RHEL 9) is applied
to the image via `oscap --remediate` with a tailoring file
([../files/stig/diskless-tailoring.xml](../files/stig/diskless-tailoring.xml)).
This register documents every **intentional** deviation — the rules unselected in
that tailoring, plus posture decisions — with the rationale and compensating
control. The per-build **generated** report
(`stig-deviations.md` / `stig-report.html`, produced by
[../scripts/stig-deviations.sh](../scripts/stig-deviations.sh) from the
post-remediation scan) cross-references this file: any non-`pass` rule **not**
listed here is flagged **REVIEW**.

Posture decisions (2026-06-18, lab phase):
- **FIPS: OFF** — revisit alongside UKI signing / Secure Boot.
- **Audit offload: LOCAL-only** — auditd runs and logs to the tmpfs overlay; no
  remote collector yet.
- **CI: report-only** — `oscap eval` publishes ARF + HTML; never fails the build.

## Intentional deviations

| # | STIG/SSG area (rules) | Why it doesn't apply / can't be met | Compensating control |
|---|---|---|---|
| 1 | **Separate partitions + their mount options** for `/var`, `/var/log`, `/var/log/audit`, `/tmp`, `/home`, `/var/tmp`, `/dev/shm` (`partition_for_*`, `mount_option_*`) | Root is a single immutable **erofs** image on NVMe/TCP with a **tmpfs volatile overlay** — there is no local disk to partition, and these dirs are not separate mounts. The related `mount_option_*` rules will therefore show as findings; they are covered by this entry. | The entire root is **read-only** (erofs+dm-verity) and the writable layer is **tmpfs (RAM)**, which structurally bounds the tampering / space-exhaustion risks the partition+option rules target. `noexec` on `/tmp` is intentionally NOT forced — the browser kiosk uses `/tmp` for its profile/cache and can break under `noexec`. |
| 2 | **Bootloader / GRUB** (`grub2_*`: password, admin user, and the `*_argument` kernel-param rules) | Boot is a **UKI** (systemd-stub) over UEFI HTTP boot — there is **no GRUB**. | The STIG kernel params (`audit=1`, `audit_backlog_limit`, `slub_debug=P`, `page_poison=1`, `vsyscall=none`, `init_on_alloc/free`) are baked into the **signed UKI cmdline** ([../scripts/extract-artifacts.sh](../scripts/extract-artifacts.sh)); the cmdline is immutable once signed (Secure Boot covers it), which is stronger than an editable GRUB config. |
| 3 | **AIDE / file integrity** (`package_aide_installed`, `aide_build_database`, `aide_periodic_cron_checking`, `aide_scan_notification`, `aide_verify_*`) | The root is read-only and cryptographically verified; an on-disk AIDE db would also be non-deterministic and break the reproducible verity hash. | **dm-verity** verifies the whole root filesystem against a signed root hash on every read — continuous, hardware-backed integrity that supersedes AIDE's periodic scan. |
| 4 | **Firewall** (`service_firewalld_enabled`, `package_firewalld_installed`, `set_firewalld_default_zone`) | Currently `firewall --disabled`; auto-enabling firewalld/nftables risks the NVMe/TCP (`:4420`) and NetworkManager paths during bring-up. | Isolated boot LAN; egress controlled at the serving-stack / network firewall layer. **Future hardening** — re-enable with explicit allow rules once the secure path is verified. |
| 5 | **FIPS** (`enable_fips_mode`, `grub2_enable_fips_mode`, `package_dracut-fips_installed`) | Decision: off for the lab phase (interacts with the still-unsigned UKI / Secure Boot story). | Revisit with signing. No weakened crypto is configured in the meantime. |
| 6 | **Remote audit offload** (`auditd_audispd_configure_remote_server`, `..._encrypt_sent_records`, `..._network_failure_action`, `rsyslog_remote_loghost`) **+ rsyslog TLS offload** (`rsyslog_encrypt_offload_defaultnetstreamdriver`, `..._actionsendstreamdrivermode`, `..._actionsendstreamdriverauthmode`) | Offload is done via **rsyslog over TCP** (simpler, container-friendly receiver); the `audisp-remote` native-protocol rules stay waived. The **TLS-offload rules are waived** because the isolated lab boot LAN is plaintext (mirrors HTTP boot / no MACsec) and applying them forces `gtls` + x509 peer auth + a CA file, which **breaks** the plaintext `2f` forward (no CA, plaintext `:514`). | **Wired both ends, plaintext.** Sender (image, provisioner 2f, `audit_host`): audisp-syslog → LOCAL6 + rsyslog `omfwd` (disk-assisted queue) → host-generic `audit.fleet.lab`. Receiver: `serving/audit-collector/` (rsyslog TCP/514, per-source-host files). No per-host state; isolated LAN. **Production (tracked in docs/TODO.md #2):** issue the collector a TLS cert, bundle the CA in the image, switch `2f` + the collector to `gtls`/6514, then **re-select** the three `rsyslog_encrypt_offload_*` rules (drop the waivers) so they pass. |
| 7 | **Smart card login** (`require_smart_card_login`, `sssd_enable_smartcard`, `install_smartcard_packages`) | Diskless hosts have no smartcard reader; admin access is SSH key-based. Enforcing smartcard login would lock the host out. | Admin auth is an SSH **key-only** account; root is locked; password auth disabled. |
| 8 | **NOPASSWD sudo** (`sudo_remove_nopasswd`) | The baked admin is a **passwordless, key-only** account, so `sudo` cannot prompt for a password — NOPASSWD is required for it to function at all. | Single fleet-generic admin, key-only, `wheel`; no interactive password exists to protect. Tracked for the eventual per-host/rotatable access model (SSH CA / config-agent). |
| 9 | **GUI / autologin / GDM** (gnome screensaver, GDM banner, automatic-login rules) | No GNOME/GDM is installed; the kiosk is an **intentional** autologin single-app (weston + Chromium) session on kiosk builds. | Kiosk user is unprivileged and passwordless; the session runs a single locked-down browser (no shell, no nav). Applies to kiosk builds only; non-kiosk builds don't install the GUI stack. |
| 10 | **Password aging / faillock on the key-only APPLIANCE ACCOUNTS** (`accounts_*_age_*`, `account_disable_post_pw_expiration`, `pam_faillock` account phase) — applied to the *system* but **exempted for the `fleetadmin` and `kiosk` users** | These two accounts are **passwordless / key-only**. The hardened PAM *account* phase (faillock + password aging/inactivity) treats a passwordless account as expired/lockable, which **locked the fleet out**: the kiosk autologin session never opened (getty kept tty1) and sshd denied the valid key with `Permission denied (publickey)` (pubkey auth succeeds, then `pam_acct_mgmt` fails). Fixed 2026-06-18. | The exemption is **scoped to these two appliance accounts only** (post-STIG `chage -M -1 -m 0 -I -1 -E -1` + `faillock --reset`; kiosk uses a dedicated `/etc/pam.d/kiosk`; sshd gets an `account sufficient pam_succeed_if.so user = fleetadmin` bypass as its first account rule). Every other account keeps the full STIG password policy + faillock. These accounts have **no password to age** and (key-only, `PasswordAuthentication no`) **no password-auth path for faillock to protect**. Folds into the per-host/rotatable access model (SSH CA / config-agent). |

| 11 | **Login banner content** (`banner_etc_issue`, `banner_etc_issue_net`, `banner_etc_motd`) | STIG requires the verbatim DoD/USG "you are accessing a U.S. Government Information System" text. This is a fictional starship fleet, not a USG system, so `/etc/issue{,.net}` + `/etc/motd` carry the fleet (ISV AURELIAN) notice instead (provisioner step 2e, `files/banner/fleet-issue`). The content rules therefore fail by design. | The banner **keeps its security function**: it still states authorized-crew-only, that the terminal is monitored/logged, and that unauthorized access is prohibited — only the wording is themed. `sshd_enable_warning_banner` (Banner is shown) still passes. |

## 12. Report triage 2026-06-19 (scan `app-9f7d8d1`)

Triage of the 21 REVIEW rows from the post-fix hardened scan (398 pass / 12 fail
/ 9 notchecked — a valid scan). All but one are dispositioned below; the rule IDs
are unselected in `diskless-tailoring.xml` (or made to pass / N/A) so they no
longer surface as REVIEW. **Remaining open: `selinux_context_elevation_for_sudo`**
(see end).

**Waived — covered by an existing register entry (Bucket A):**
- `encrypt_partitions`, `auditd_audispd_configure_sufficiently_large_partition` —
  single erofs+tmpfs root, no partitions (#1).
- `grub2_password` — UKI, no GRUB (#2).
- `sysctl_crypto_fips_enabled`, `harden_sshd_macs_openssh_conf_crypto_policy`,
  `harden_sshd_macs_opensshserver_conf_crypto_policy` — FIPS off (#5).
- `configured_firewalld_default_deny` — firewall off for the lab boot LAN (#4).
- `sssd_enable_certmap`, `sssd_has_trust_anchor` — no SSSD/PKI/smartcard; local
  key-only host, no domain join (#7).

**Waived — architecturally absent, N/A (Bucket B):** `account_temp_expire_date`
(no temporary accounts), `accounts_user_home_paths_only` (appliance accounts,
fixed home layout), `libreswan_approved_tunnels` (no IPsec/libreswan),
`security_patches_up_to_date` (rebuild-from-current model — a scanner can't assess
patch currency against a future rebuild), `ssh_keys_passphrase_protected` (no
private keys stored on the host; host keys regenerate per boot).

**Waived — decisions (Bucket C):**
- `installed_OS_is_vendor_supported` — Rocky **is** vendor-supported; the rule
  fails only because the RHEL datastream's CPE checks for an RHEL subscription.
- `network_configure_name_resolution` — single DHCP-provided resolver by design
  on the boot LAN; the multi-DNS requirement is N/A to the netboot model.
- `sysctl_user_max_user_namespaces_no_remediation` — **KEEP user namespaces
  enabled**: the Chromium kiosk sandbox requires them. Disabling userns would
  break the kiosk.
- `accounts_authorized_local_users` — the authorized-users check doesn't know the
  key-only appliance accounts (`fleetadmin`, `kiosk`); they are authorized (see
  #10). *Production note: prefer setting the SSG authorized-users regex variable
  to assert these two explicitly rather than waiving the whole check.*
- `package_postfix_installed` — **postfix removed** (provisioner 2c-STIG): the
  appliance sends no mail and ships audit off-box via rsyslog (2f), so a local
  MTA is dead attack surface. This also makes `postfix_prevent_unrestricted_relay`
  N/A. Audit-failure notification relies on the remote offload, not email
  (`auditd ... action_mail_acct` stays configured but is not the alerting path).
- `auditd_data_retention_space_left_action` — **consequence of removing postfix.**
  STIG sets `space_left_action = email`, but with no MTA auditd logs *"Email
  option is specified but /usr/lib/sendmail doesn't seem executable"* at every
  boot and can't send. 2c-STIG repoints **`space_left_action = SYSLOG`** so the
  75%-full alert rides the remote audit offload (2f) instead; this rule wants
  `email`, so it's waived. `admin_space_left_action` stays `single` (the critical
  action, unaffected) and `action_mail_acct = root` stays set.

**Remediated to pass (not waived):**
- `configure_usbguard_auditbackend` + `usbguard_allow_hid` / `usbguard_allow_hub`
  / `usbguard_allow_hid_and_hub` — **decision: ENABLE USBGuard with enforcement.**
  usbguard was already installed + enabled, but its policy is generated on the
  build VM (no real USB), so under the default **block** target a physical console
  keyboard/mouse would be blocked. The provisioner now sets
  `AuditBackend=LinuxAudit` and writes a **hardware-independent HID + hub
  allowlist** (`allow with-interface match-all { 03:*:* }`, `{ 09:00:* }`, and the
  composite `{ 03:*:* 09:00:* }`) so console input survives while mass storage and
  other device classes are blocked. **⚠ BENCH-VERIFY physical console input on
  real hardware** — if a platform's input device doesn't match the HID/hub
  classes, fall back to a per-platform allow rule or re-waive
  `configure_usbguard_auditbackend`.

**Remediated with confinement (decision 2026-06-20) — `selinux_context_elevation_for_sudo`:**
SSG ships no automated fix; the STIG manual remediation
(`... TYPE=sysadm_t ROLE=sysadm_r ...`) requires the admin to reach the
`sysadm_r` role, which `unconfined_u` cannot. So the provisioner (2c-STIG) now
(1) rewrites the admin sudoers rule to `<admin> ALL=(ALL) TYPE=sysadm_t
ROLE=sysadm_r NOPASSWD:ALL` (keeps NOPASSWD — the admin is key-only, #8 — and is
what the OVAL check matches), and (2) maps the admin to the confined **`staff_u`**
SELinux user (`semanage login`) so the `staff_r`→`sysadm_r` sudo transition is
authorized at runtime. `staff_u` (not `sysadm_u`) is used so SSH login isn't gated
by the `ssh_sysadm_login` boolean. **⚠ Two bench gates before trusting:** (a)
admin SSH login + `sudo` + every privileged op must work under `staff_u`
confinement — if it locks out, `enable_stig=false` is the escape hatch and this
reverts to a justified deviation; (b) reproducibility — `semanage` rebuilds the
SELinux policy store, so confirm the verity hash is stable across two builds (and
normalize in 2d hygiene if it drifts).

## 13. Ctrl+Alt+Del console reboot hotkey kept enabled (decision 2026-06-20)

The RHEL 9 STIG disables the console reboot hotkey via two rules, both **waived**
in the tailoring:
- `disable_ctrlaltdel_reboot` — masks `ctrl-alt-del.target` (single Ctrl+Alt+Del).
- `disable_ctrlaltdel_burstaction` — sets `CtrlAltDelBurstAction=none` in
  `/etc/systemd/system.conf` (the rapid 7× burst reboot).

**Why waived:** the STIG intent is to stop an **unauthenticated console user from
causing an unplanned reboot/outage**. On this fleet that risk is structurally
absent — the hosts are **diskless and stateless** (immutable erofs+dm-verity root,
tmpfs volatile overlay), so a console reboot loses nothing and persists nothing,
and a **field technician needs to recover a wedged kiosk from the keyboard when
SSH is unavailable**. The two rules are unselected so `--remediate` leaves the
systemd default (the hotkey reboots).

**Compensating control / scope:** the hotkey only triggers a **clean stateless
reboot** — it cannot exfiltrate data, alter the read-only root, or bypass auth to
a shell. Authenticated reboot paths (`systemctl reboot` over the key-only admin
SSH session) are unaffected either way. The console itself is otherwise locked
down (kiosk autologin to a single locked browser; no getty shell on tty1;
`debug_shell` is lab-only and never shipped). If a future deployment wants the
hotkey disabled (e.g. a physically exposed terminal), re-select both rule IDs.

## 14. Local SELinux policy module `chrony_dhcp_dispatcher` (added 2026-07-03)

Not a STIG-rule waiver — a **minimal local `allow` module** shipped to keep a
legitimate function working under enforcing SELinux. Source:
`files/stig/chrony-dhcp-dispatcher.te`; built + `semodule -i`'d in the 2c-STIG
provisioner.

**The one rule:** `allow chronyd_t NetworkManager_dispatcher_custom_t:unix_dgram_socket sendto;`

**Why:** the NM dispatcher script `10-chrony-dhcp-ntp` loads DHCP option-42 NTP
servers by running `chronyc reload sources`. That `chronyc` runs in the confined
custom-dispatcher domain (`NetworkManager_dispatcher_custom_t`), so its reply
socket carries that label; the shipped policy has no rule for `chronyd_t` to
`sendto` it. Under enforcing, chronyd processes the reload but its **ack is
denied** — `chronyc` reports failure and the AVC floods the SIEM (seen
2026-07-03). An **`allow`** (not `dontaudit`) is used deliberately: it both stops
the noise and lets the reload ack succeed, so `chronyc` returns cleanly.

**Scope:** one direction, one class/perm, two well-known types. It grants no
network/file access and nothing beyond the chronyd↔dispatcher reply the DHCP-NTP
path needs. **BENCH-VERIFY:** (a) on an *enforcing* boot the AVC is gone and
`chronyc reload sources` from the dispatcher returns 0 (confirm a DHCP-handed NTP
server appears in `chronyc sources`); (b) no follow-on AVC (extend the `.te` if
one shows); (c) **reproducibility** — `semodule -i` rebuilds the policy store, so
re-check the verity hash is stable across two builds (same concern as the
`semanage` step, #12/sudo elevation). NB: only shipped when `enable_stig=true`
(the module install lives in the STIG-gated provisioner); a non-STIG *enforcing*
build would still see the AVC.

## 15. Local SELinux policy module `fapolicyd_tmpfs` (added 2026-07-03)

Not a STIG-rule waiver — a **minimal local `allow` module** shipped to close an
**exec-guard gap** under enforcing SELinux. Source: `files/stig/fapolicyd-tmpfs.te`;
built + `semodule -i`'d in the 2c-STIG provisioner (right after #14).

**The one rule:** `allow fapolicyd_t tmpfs_t:filesystem getattr;`

**Why:** fapolicyd is the app-allowlisting **exec guard for the writable RAM
overlay** — dm-verity covers only the read-only erofs lower, so a binary dropped
into the tmpfs overlay upper is exactly what fapolicyd must catch (this is why we
KEEP fapolicyd; see "Things that are NOT deviations"). fapolicyd `statfs`'s mounts
to decide which to watch, and **tmpfs is in its default `watch_fs`**. This image's
root is `systemd.volatile=overlay`, so `statfs("/")` returns the overlay **upper**
— a tmpfs (audit: `dev=tmpfs ino=1 name="/"`, `fapolicyd_t → tmpfs_t:filesystem
getattr`, `permissive=0`, seen 2026-07-03). The shipped policy has no such rule
(no ordinary host has its root on tmpfs), so under enforcing the `getattr` is
**denied**, fapolicyd cannot fully mark the tmpfs mounts (`/`, `/tmp`, `/dev/shm`),
and its guard over the writable overlay is **incomplete** — a real gap, not just
SIEM noise. An **`allow`** (not `dontaudit`): we *want* fapolicyd to see the tmpfs.

**Scope:** one perm (`getattr`) on one class (`filesystem`), two well-known types.
Grants no file read/write, only the `statfs` fapolicyd needs to classify + watch
the mount. **BENCH-VERIFY:** (a) the AVC is gone on an enforcing boot; (b) fapolicyd
**blocks a non-RPM executable dropped in `/tmp`** (proves the guard now covers the
overlay tmpfs) while the trusted kiosk/appliance binaries still run; (c)
**reproducibility** — `semodule -i` rebuilds the policy store, so re-check verity
hash stability across two builds (same concern as #12/#14). NB: only shipped when
`enable_stig=true`; a non-STIG enforcing build would still see the AVC and the gap.

## Things that are NOT deviations (kept / enforced)

- **SELinux enforcing** (`selinux --enforcing`; labels preserved into the erofs).
- **fapolicyd** kept **enabled** (NOT waived) — it's the real exec-allowlist guard
  for the writable RAM overlay, which dm-verity does **not** cover. The only
  adjustment: the few non-RPM executables we ship are added to fapolicyd's trust DB
  at build (provisioner 2c-STIG-postfix) so they run; everything else stays
  RPM-trust-only. Trusted custom binaries: `/usr/local/bin/kiosk-session`,
  `/usr/local/bin/fleet-sb-enroll`, `/usr/local/bin/fleet-hostapi` (the loopback
  host-info agent, built from `files/hostapi`). Each is built into the immutable,
  dm-verity-protected root, so trusting it doesn't weaken the overlay exec guard.
  (fapolicyd blocking `kiosk-session` was the original kiosk-won't-start cause.)
- **auditd** enabled; STIG audit rules applied; logs locally.
- **USBGuard** enabled + enforcing (`block` target) with a HID/hub allowlist and
  `AuditBackend=LinuxAudit` (see #12) — blocks USB mass storage / unknown device
  classes at the console while keeping keyboard/mouse working.
- **sshd hardening**: root login off, password & keyboard-interactive auth off,
  key-only (drop-in `50-fleet-hardening.conf`); STIG ciphers/MACs/KEX applied.
- **No per-host state**: SSH host keys stripped at build, regenerated at first
  boot into the overlay; `machine-id` empty.
- Password-policy / faillock rules remain **selected and applied system-wide**
  (kept honest in the report); they are exempted only for the two key-only
  appliance accounts — see deviation #10.

## How to read the generated report

`scripts/stig-deviations.sh stig-arf.xml` emits a table of every rule whose
result is not `pass`. Each row is tagged:
- **intentional** — matches an unselected rule / area in this register.
- **REVIEW** — a real finding to triage (fix it, or add a justified entry here).