harden(install): tag the journalctl sudo grants NOEXEC

journalctl starts a pager when its output is a terminal, and from less a "!sh"
is a shell with whatever privileges journalctl was given. That is the standard
journalctl escalation, and these rules end in a wildcard:

    <user> ALL=(ALL) NOPASSWD: /usr/bin/journalctl -u ledmatrix *

Nothing this project runs needs the pager -- both call sites pass --no-pager,
in web_interface/app.py and api_v3.py. But a sudoers rule cannot require a flag
that sits in the middle of a command line, and reasoning about what a trailing
wildcard does and does not admit is exactly the kind of subtlety that produces
a hole. sudo's NOEXEC tag stops the command executing another program at all,
which closes it without depending on that reasoning.

NOEXEC works by LD_PRELOAD, so it applies to dynamically linked binaries.
Checked on the target hardware: journalctl there is dynamically linked. The
generated rules were run through `visudo -c` -- parsed OK.

Found while auditing the pre-existing wildcard grants, prompted by review
catching a far worse one I had added myself in the same area: `iptables *`,
where --modprobe runs an arbitrary path as root.

Reachability, stated plainly: on a stock Raspberry Pi image none of this
matters, because 010_pi-nopasswd already grants the default user
`ALL=(ALL) NOPASSWD: ALL`. It matters on a hardened install, or where the
service runs as a user without that blanket rule.

Two mutation checks: dropping NOEXEC from a rule fails, and deleting the rules
rather than tagging them fails too -- that second one matters, since "make the
test pass" and "remove the feature" would otherwise look the same.

first_time_install.sh

@@ -1419,9 +1419,16 @@ $ACTUAL_USER ALL=(ALL) NOPASSWD: $BASH_PATH $PROJECT_ROOT_DIR/scripts/fix_perms/
 EOF
 if [ -n "$JOURNALCTL_PATH" ]; then
     cat >> /tmp/ledmatrix_web_sudoers << EOF
-$ACTUAL_USER ALL=(ALL) NOPASSWD: $JOURNALCTL_PATH -u ledmatrix.service *
-$ACTUAL_USER ALL=(ALL) NOPASSWD: $JOURNALCTL_PATH -u ledmatrix *
-$ACTUAL_USER ALL=(ALL) NOPASSWD: $JOURNALCTL_PATH -t ledmatrix *
+# NOEXEC, because these rules end in a wildcard and journalctl starts a pager
+# when its output is a terminal. From that pager (less) a "!sh" is a root
+# shell -- the standard journalctl escalation. The web interface always passes
+# --no-pager, so nothing here needs it, but the rule cannot require a flag that
+# sits in the middle of the command line. NOEXEC stops the command executing
+# another program at all, which closes the hole without depending on wildcard
+# matching subtleties.
+$ACTUAL_USER ALL=(ALL) NOPASSWD:NOEXEC: $JOURNALCTL_PATH -u ledmatrix.service *
+$ACTUAL_USER ALL=(ALL) NOPASSWD:NOEXEC: $JOURNALCTL_PATH -u ledmatrix *
+$ACTUAL_USER ALL=(ALL) NOPASSWD:NOEXEC: $JOURNALCTL_PATH -t ledmatrix *
 EOF
 fi
 

test/test_sudoers_noexec_on_pagers.py

@@ -0,0 +1,87 @@
+"""Wildcard grants to commands that start a pager must carry NOEXEC.
+
+`journalctl` runs a pager when its output is a terminal, and from `less` a
+`!sh` is a shell with the privileges journalctl was given. That is the standard
+journalctl privilege escalation, and the installer's rules end in a wildcard:
+
+    <user> ALL=(ALL) NOPASSWD: /usr/bin/journalctl -u ledmatrix *
+
+The web interface always passes --no-pager -- both call sites do, in app.py and
+api_v3.py -- so nothing the project runs needs the pager. But a sudoers rule
+cannot require a flag that sits in the middle of the command line, and reasoning
+about what a trailing `*` does or does not admit is exactly the kind of
+subtlety that produces a hole.
+
+sudo's NOEXEC tag stops the command executing another program at all, which
+closes it without depending on that reasoning. It works by LD_PRELOAD, so it
+applies to dynamically linked binaries; journalctl is one.
+
+On a stock Raspberry Pi image none of this is reachable, because
+/etc/sudoers.d/010_pi-nopasswd already grants the default user
+`ALL=(ALL) NOPASSWD: ALL`. It matters on a hardened install, or where the
+service runs as a user without that blanket rule.
+"""
+import re
+from pathlib import Path
+
+import pytest
+
+ROOT = Path(__file__).resolve().parent.parent
+INSTALLERS = (
+    ROOT / "first_time_install.sh",
+    ROOT / "scripts" / "install" / "configure_wifi_permissions.sh",
+)
+
+#: Commands that will start another program of their own accord -- a pager, an
+#: editor, a shell -- and so must not be granted the ability to do so.
+SPAWNS_A_PROGRAM = ("journalctl", "systemctl", "less", "more", "man", "git")
+
+
+def _grant_lines():
+    lines = []
+    for installer in INSTALLERS:
+        if not installer.is_file():
+            continue
+        for line in installer.read_text(encoding="utf-8", errors="replace").splitlines():
+            stripped = line.strip()
+            if "NOPASSWD" in stripped and not stripped.startswith("#"):
+                lines.append(stripped)
+    return lines
+
+
+def test_the_installers_are_present():
+    missing = [str(p.relative_to(ROOT)) for p in INSTALLERS if not p.is_file()]
+    assert not missing, f"installer(s) missing: {missing}"
+
+
+def test_wildcard_pager_grants_carry_noexec():
+    offenders = []
+    for rule in _grant_lines():
+        command = rule.split("NOPASSWD", 1)[1]
+        if not command.rstrip().endswith("*"):
+            continue
+        tool = command.replace("_PATH", "").replace("$", "").lower()
+        for name in SPAWNS_A_PROGRAM:
+            if re.search(rf"(^|/|\s){name}(\s|$)", tool):
+                if "NOEXEC" not in rule:
+                    offenders.append(rule)
+                break
+    assert not offenders, (
+        "wildcard grant to a command that can start a pager or shell, without "
+        "NOEXEC:\n  " + "\n  ".join(offenders))
+
+
+def test_journalctl_is_granted_at_all():
+    """Guard against 'fixing' the above by deleting the rules."""
+    text = "\n".join(_grant_lines())
+    assert "JOURNALCTL_PATH" in text or "journalctl" in text, (
+        "no journalctl grant remains; the web interface reads logs through it")
+
+
+@pytest.mark.parametrize("unit", ["ledmatrix.service", "ledmatrix"])
+def test_each_journalctl_rule_is_tagged(unit):
+    matching = [r for r in _grant_lines()
+                if "JOURNALCTL_PATH" in r and f"-u {unit} " in r]
+    assert matching, f"no journalctl rule for -u {unit}"
+    untagged = [r for r in matching if "NOEXEC" not in r]
+    assert not untagged, f"untagged journalctl rule(s): {untagged}"