Add systemd unit posture checks

Close the remaining v0.8 posture bullet: flag enabled systemd units with
group/world-writable unit files (systemd_unit_writable, sid 100050) and
units whose ExecStart binary runs from a writable/unsafe path such as
/tmp, /dev/shm, /var/tmp, /home, or /run/user (systemd_exec_unsafe_path,
sid 100051).

- posture.systemd_findings/parse_systemctl_show/read_systemd_units, wired
  into posture.run() behind the new posture_systemd config knob.
- read_systemd_units shells out to systemctl with timeouts and fails closed
  on non-systemd hosts; the dashboard /api/posture surfaces findings for free.
- Unit tests for the parser and evaluator; TestRunIntegration patches the
  new reader so run() never touches live systemctl.
- Docs: COMMAND_REFERENCE posture list, ROADMAP v0.8 bullet, RUNBOOKS
  Runbook 2 triggers, config TOML knob.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

enodia_sentinel/posture.py

@@ -19,13 +19,16 @@ from __future__ import annotations
 
 import glob
 import os
+import re
 import stat
 from collections.abc import Iterable, Iterator
+from typing import NamedTuple
 
 from .alert import Alert, Severity
 from .config import Config
 
-# --- SID allocation (host-posture: 100040-100049) -------------------------
+# --- SID allocation (host-posture: 100040-100047, 100050-100051;
+#     100048-100049 belong to the memory_obfuscation detector) -------------
 SID_SSH_ROOT_LOGIN = 100040
 SID_SSH_PASSWORD_AUTH = 100041
 SID_SSH_EMPTY_PASSWORD = 100042
@@ -34,6 +37,8 @@ SID_SUDO_NOAUTH = 100044
 SID_SUDOERS_WRITABLE = 100045
 SID_PATH_WRITABLE = 100046
 SID_SENSITIVE_FILE_PERM = 100047
+SID_SYSTEMD_UNIT_WRITABLE = 100050
+SID_SYSTEMD_EXEC_UNSAFE = 100051
 
 CLASSTYPE = "host-posture"
 
@@ -42,6 +47,11 @@ _DEFAULT_PATH_DIRS = (
     "/sbin", "/bin", "/root/bin",
 )
 
+# An enabled service whose ExecStart binary lives under one of these is a
+# persistence/hijack red flag — these are user- or world-writable locations a
+# system service should never run from.
+_UNSAFE_EXEC_PREFIXES = ("/tmp/", "/dev/shm/", "/var/tmp/", "/home/", "/run/user/")
+
 # (path, world-readable severity, group/world-writable severity). None = skip
 # that rule for this file. Keyed by the kind of secret each file holds.
 _SENSITIVE_FILES: tuple[tuple[str, Severity | None, Severity | None], ...] = (
@@ -281,6 +291,108 @@ def read_sensitive_files(
     return out
 
 
+# --- systemd units --------------------------------------------------------
+
+class UnitFact(NamedTuple):
+    name: str
+    fragment_path: str
+    fragment_mode: int          # st_mode of the unit file (0 if unknown)
+    enabled: bool
+    exec_paths: tuple[str, ...]  # ExecStart binary paths
+
+
+def systemd_findings(units: Iterable[UnitFact]) -> Iterator[Alert]:
+    """Findings from enabled systemd units.
+
+    Flags a unit file anyone can rewrite (group/world-writable — what root runs
+    at boot becomes attacker-controlled) and an enabled service whose ExecStart
+    binary lives in a writable/unsafe directory (a classic systemd persistence
+    trick).
+    """
+    for u in units:
+        if u.fragment_path and (u.fragment_mode & 0o022):
+            yield Alert(
+                severity=Severity.HIGH, signature="systemd_unit_writable",
+                key=f"posture:systemd:writable:{u.name}",
+                detail=f"systemd unit file is group/world-writable "
+                       f"({stat.filemode(u.fragment_mode)}): {u.fragment_path} ({u.name})",
+                sid=SID_SYSTEMD_UNIT_WRITABLE, classtype=CLASSTYPE)
+        for path in u.exec_paths:
+            if path.startswith(_UNSAFE_EXEC_PREFIXES):
+                yield Alert(
+                    severity=Severity.HIGH, signature="systemd_exec_unsafe_path",
+                    key=f"posture:systemd:exec:{u.name}:{path}",
+                    detail=f"enabled systemd unit {u.name} runs from a writable/unsafe "
+                           f"path: {path}",
+                    sid=SID_SYSTEMD_EXEC_UNSAFE, classtype=CLASSTYPE)
+
+
+_EXEC_PATH_RE = re.compile(r"path=(\S+)")
+
+
+def parse_systemctl_show(text: str) -> list[tuple[str, str, bool, tuple[str, ...]]]:
+    """Parse ``systemctl show`` output into (name, fragment_path, enabled, execs).
+
+    Pure: blocks are separated by blank lines; each carries ``Id``,
+    ``FragmentPath``, ``UnitFileState``, and zero or more ``ExecStart`` lines
+    (whose ``path=`` field is the resolved binary). No filesystem access.
+    """
+    out: list[tuple[str, str, bool, tuple[str, ...]]] = []
+    for block in text.split("\n\n"):
+        props: dict[str, str] = {}
+        execs: list[str] = []
+        for line in block.splitlines():
+            key, _, val = line.partition("=")
+            if key == "ExecStart":
+                m = _EXEC_PATH_RE.search(val)
+                if m:
+                    execs.append(m.group(1))
+            else:
+                props.setdefault(key, val)
+        name = props.get("Id", "")
+        if not name:
+            continue
+        out.append((name, props.get("FragmentPath", ""),
+                    props.get("UnitFileState", "") == "enabled", tuple(execs)))
+    return out
+
+
+def read_systemd_units(systemctl: str = "systemctl") -> list[UnitFact]:
+    """Gather enabled systemd service units as UnitFacts (fails closed).
+
+    Non-systemd or restricted hosts (no ``systemctl``) yield no facts.
+    """
+    import subprocess
+    try:
+        listing = subprocess.run(
+            [systemctl, "list-unit-files", "--type=service",
+             "--state=enabled", "--no-legend", "--no-pager"],
+            capture_output=True, text=True, timeout=8).stdout
+    except (OSError, subprocess.SubprocessError):
+        return []
+    names = [line.split()[0] for line in listing.splitlines() if line.split()]
+    if not names:
+        return []
+    try:
+        shown = subprocess.run(
+            [systemctl, "show", "--no-pager", "--property=Id",
+             "--property=FragmentPath", "--property=ExecStart",
+             "--property=UnitFileState", *names],
+            capture_output=True, text=True, timeout=12).stdout
+    except (OSError, subprocess.SubprocessError):
+        return []
+    facts: list[UnitFact] = []
+    for name, frag, enabled, execs in parse_systemctl_show(shown):
+        mode = 0
+        if frag:
+            try:
+                mode = os.stat(frag).st_mode
+            except OSError:
+                mode = 0
+        facts.append(UnitFact(name, frag, mode, enabled, execs))
+    return facts
+
+
 # --- orchestration --------------------------------------------------------
 
 def run(cfg: Config) -> Iterator[Alert]:
@@ -293,6 +405,9 @@ def run(cfg: Config) -> Iterator[Alert]:
     yield from path_findings(read_path_dirs(path_dirs))
     yield from sensitive_file_findings(read_sensitive_files())
 
+    if cfg.posture_systemd:
+        yield from systemd_findings(read_systemd_units())
+
     # Package-signature policy is a posture question too; reuse the tamper check.
     from . import pkgdb
     sl = pkgdb.siglevel_alert()