Fix: Gate SSH hardening on a verified key
This commit is contained in:
Gabriel Brown
@@ -33,6 +33,60 @@ for arg in "$@"; do
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esac
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done
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# The public bootstrap contract runs this branch as an ordinary user with a
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# stubbed root identity. Keep its filesystem adapter unavailable to a real root
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# shell so it cannot redirect a real installation by accident.
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BOOT_ROOT="${PANAMA_BOOT_FIXTURE_ROOT:-}"
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if [[ -n "$BOOT_ROOT" && "$EUID" -eq 0 ]]; then
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echo "boot: PANAMA_BOOT_FIXTURE_ROOT is test-only" >&2
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exit 1
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fi
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system_path() {
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local path="$1"
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[[ "$path" == /* ]] || return 2
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printf '%s%s\n' "$BOOT_ROOT" "$path"
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}
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safe_authorized_keys() {
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local username="$1" user_home="$2" uid ssh_dir keys
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uid="$(id -u "$username")" || return 1
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[[ "$uid" =~ ^[0-9]+$ && "$uid" != 0 && "$user_home" == /* ]] || return 1
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ssh_dir="$user_home/.ssh"
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keys="$ssh_dir/authorized_keys"
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[[ -d "$ssh_dir" && ! -L "$ssh_dir" && -f "$keys" && ! -L "$keys" ]] || return 1
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[[ "$(stat -Lc '%u:%a' "$ssh_dir")" == "$uid:700" ]] || return 1
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[[ "$(stat -Lc '%u:%a' "$keys")" == "$uid:600" ]] || return 1
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grep -qEv '^[[:space:]]*(#|$)' "$keys"
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}
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safe_root_authorized_keys() {
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local keys
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keys="$(system_path /root/.ssh/authorized_keys)" || return 1
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[[ -f "$keys" && ! -L "$keys" ]] || return 1
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[[ "$(stat -Lc '%u:%a' "$keys")" == '0:600' ]] || return 1
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grep -qEv '^[[:space:]]*(#|$)' "$keys"
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}
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harden_server_ssh() {
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local username="$1" user_home="$2" sshd_dir sshd_dropin harden
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sshd_dir="$(system_path /etc/ssh/sshd_config.d)" || return 1
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sshd_dropin="$sshd_dir/90-panama.conf"
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if [[ -f "$sshd_dropin" ]]; then
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echo "sshd is already hardened ($sshd_dropin)"
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return 0
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fi
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printf 'Harden sshd (disable root login and password auth)? [Y/n]: '
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read -r harden </dev/tty || harden=""
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if [[ ! "$harden" =~ ^[Nn] ]]; then
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printf 'PermitRootLogin no\nPasswordAuthentication no\n' >"$sshd_dropin"
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systemctl reload sshd 2>/dev/null || systemctl reload ssh 2>/dev/null || true
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echo "Wrote $sshd_dropin; make sure your key works before logging out."
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fi
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}
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# Panama assumes Fedora's repositories and package names.
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if ! grep -qi '^ID=fedora' /etc/os-release 2>/dev/null; then
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echo "This looks like something other than Fedora; Panama only supports Fedora." >&2
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@@ -84,33 +138,46 @@ if [[ "$(id -u)" -eq 0 ]]; then
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passwd "$username" </dev/tty
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fi
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# The key that reached root is the key that should reach the user, or the
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# next SSH login has no way in once root logins are closed below.
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user_home="$(getent passwd "$username" | cut -d: -f6)"
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if [[ -s /root/.ssh/authorized_keys && ! -s "$user_home/.ssh/authorized_keys" ]]; then
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echo "Copying root's authorized_keys to $username"
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mkdir -p "$user_home/.ssh"
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cp /root/.ssh/authorized_keys "$user_home/.ssh/authorized_keys"
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chmod 700 "$user_home/.ssh"
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chmod 600 "$user_home/.ssh/authorized_keys"
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chown -R "$username:$username" "$user_home/.ssh"
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# Do not close root/password access until the account's key is an exact,
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# usable login path. The fixture adapter resolves these logical system paths
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# beneath a temporary root; ordinary execution receives the original paths.
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logical_user_home="$(getent passwd "$username" | cut -d: -f6)"
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user_home=""
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if [[ "$logical_user_home" == /* ]]; then
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user_home="$(system_path "$logical_user_home")" || true
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fi
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bootstrap_home="$user_home"
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if [[ -z "$bootstrap_home" ]]; then
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bootstrap_home="$(system_path "/home/$username")"
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fi
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# Offered rather than imposed, defaulting to yes: a VPS keeps its provider's
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# web console, so locking password and root logins out of sshd is
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# recoverable even when it goes wrong. Written as a drop-in so it never
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# fights the distribution's own sshd_config.
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SSHD_DROPIN=/etc/ssh/sshd_config.d/90-panama.conf
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if [[ -f "$SSHD_DROPIN" ]]; then
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echo "sshd is already hardened ($SSHD_DROPIN)"
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else
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printf 'Harden sshd (disable root login and password auth)? [Y/n]: '
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read -r harden </dev/tty || harden=""
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if [[ ! "$harden" =~ ^[Nn] ]]; then
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printf 'PermitRootLogin no\nPasswordAuthentication no\n' >"$SSHD_DROPIN"
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systemctl reload sshd 2>/dev/null || systemctl reload ssh 2>/dev/null || true
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echo "Wrote $SSHD_DROPIN; make sure your key works before logging out."
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user_ssh_dir="$user_home/.ssh"
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user_keys="$user_ssh_dir/authorized_keys"
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if [[ -n "$user_home" && ! -e "$user_keys" && ! -L "$user_keys" \
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&& ! -L "$user_ssh_dir" ]] && safe_root_authorized_keys; then
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copy_root_key=0
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if [[ ! -e "$user_ssh_dir" ]]; then
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mkdir -p "$user_ssh_dir"
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copy_root_key=1
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elif [[ ! -d "$user_ssh_dir" \
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|| "$(stat -Lc '%u:%a' "$user_ssh_dir")" != "$(id -u "$username"):700" ]]; then
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echo "SSH hardening unavailable: $username has no safe authorized_keys" >&2
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else
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copy_root_key=1
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fi
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if (( copy_root_key )); then
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echo "Copying root's authorized_keys to $username"
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cp "$(system_path /root/.ssh/authorized_keys)" "$user_keys"
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chmod 700 "$user_ssh_dir"
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chmod 600 "$user_keys"
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chown "$username:$username" "$user_ssh_dir" "$user_keys"
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fi
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fi
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if safe_authorized_keys "$username" "$user_home"; then
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harden_server_ssh "$username" "$user_home"
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else
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echo "SSH hardening unavailable: $username has no safe authorized_keys" >&2
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fi
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if ! command -v git >/dev/null 2>&1; then
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@@ -121,13 +188,13 @@ if [[ "$(id -u)" -eq 0 ]]; then
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# Cloned straight into the user's home and owned by them: this is the
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# checkout `panama update` will pull from for the life of the machine, and
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# a root-owned .git in a user's home is a wound that never heals.
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PANAMA_PATH="$user_home/.local/share/Panama"
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PANAMA_PATH="$bootstrap_home/.local/share/Panama"
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if [[ -d "$PANAMA_PATH/.git" ]]; then
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echo "Panama is already cloned at $PANAMA_PATH; updating"
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runuser -u "$username" -- git -C "$PANAMA_PATH" pull --ff-only \
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|| echo "Could not fast-forward; installing from the clone as it is" >&2
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else
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runuser -u "$username" -- mkdir -p "$user_home/.local/share"
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runuser -u "$username" -- mkdir -p "$bootstrap_home/.local/share"
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runuser -u "$username" -- git clone "$REPO_URL" "$PANAMA_PATH"
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fi
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@@ -271,6 +271,8 @@ hermetic tests/setup/package-lists-contract
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hermetic tests/setup/projects-contract
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hermetic tests/setup/readme-contract
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hermetic tests/setup/role-contract
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# Root bootstrap runs entirely against a temporary filesystem and PATH adapters.
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hermetic tests/setup/root-server-bootstrap-contract
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hermetic tests/setup/skills-contract
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hermetic tests/setup/test-runner-contract
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hermetic tests/setup/update-command-contract
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Executable
+300
@@ -0,0 +1,300 @@
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#!/usr/bin/env bash
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# `boot --server` is deliberately public and must be safe before it reaches the
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# cloned repository. Exercise its root branch through a PTY, against only a
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# temporary filesystem and PATH adapters.
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set -uo pipefail
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repo_dir="$(cd "$(dirname "${BASH_SOURCE[0]}")/../.." && pwd)"
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boot="$repo_dir/boot"
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[[ -x "$boot" ]] || {
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printf 'root server bootstrap: %s is not executable\n' "$boot" >&2
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exit 1
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}
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python3 - "$boot" <<'PY'
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import atexit
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import errno
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import fcntl
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import os
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import pty
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import shutil
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import subprocess
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import sys
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import tempfile
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import termios
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from pathlib import Path
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boot = sys.argv[1]
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work = Path(tempfile.mkdtemp())
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atexit.register(shutil.rmtree, work, ignore_errors=True)
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findings: list[str] = []
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def note(message: str) -> None:
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findings.append(message)
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def write_executable(path: Path, contents: str) -> None:
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path.write_text(contents)
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path.chmod(0o755)
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def make_stubs(stub_dir: Path, fixture_root: Path, calls: Path) -> None:
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common = f'''#!/usr/bin/env bash
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set -u
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calls={str(calls)!r}
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log() {{
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local argument
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{{ for argument in "$@"; do printf '%q ' "$argument"; done; printf '\\n'; }} >>"$calls"
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}}
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'''
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write_executable(stub_dir / "id", common + r'''
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log id "$@"
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case "${1:-}" in
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-u)
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case "${2:-}" in
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'') printf '0\n' ;;
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root) printf '0\n' ;;
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gib) cat "$PANAMA_BOOT_FIXTURE_ROOT/state/target-uid" ;;
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*) exit 97 ;;
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esac
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;;
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-nG) [[ "${2:-}" == gib ]] || exit 97; printf 'gib wheel\n' ;;
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*) exit 97 ;;
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esac
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''')
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write_executable(stub_dir / "passwd", common + r'''
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log passwd "$@"
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[[ "${1:-}" == -S && "${2:-}" == gib ]] || exit 97
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printf 'gib PS\n'
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''')
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write_executable(stub_dir / "getent", common + r'''
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log getent "$@"
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[[ "${1:-}" == passwd && "${2:-}" == gib ]] || exit 97
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home="$(<"$PANAMA_BOOT_FIXTURE_ROOT/state/home")"
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printf 'gib:x:1000:1000::%s:/bin/bash\n' "$home"
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''')
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write_executable(stub_dir / "stat", common + r'''
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log stat "$@"
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[[ "${1:-}" == -Lc && "${2:-}" == '%u:%a' ]] || exit 97
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case "${3:-}" in
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"$PANAMA_BOOT_FIXTURE_ROOT/home/gib/.ssh") cat "$PANAMA_BOOT_FIXTURE_ROOT/state/target-dir-meta" ;;
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"$PANAMA_BOOT_FIXTURE_ROOT/home/gib/.ssh/authorized_keys") cat "$PANAMA_BOOT_FIXTURE_ROOT/state/target-key-meta" ;;
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"$PANAMA_BOOT_FIXTURE_ROOT/root/.ssh/authorized_keys") cat "$PANAMA_BOOT_FIXTURE_ROOT/state/root-key-meta" ;;
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*) exit 97 ;;
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esac
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''')
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write_executable(stub_dir / "runuser", common + r'''
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log runuser "$@"
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[[ "${1:-}" == -u && "${2:-}" == gib && "${3:-}" == -- ]] || exit 97
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shift 3
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"$@"
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''')
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write_executable(stub_dir / "git", common + r'''
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log git "$@"
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case "${1:-}" in
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clone)
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mkdir -p "$3/.git"
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cp "$PANAMA_BOOT_FIXTURE_ROOT/stub-install" "$3/install"
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chmod +x "$3/install"
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;;
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-C) [[ "${3:-}" == pull && "${4:-}" == --ff-only ]] || exit 97 ;;
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*) exit 97 ;;
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esac
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''')
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write_executable(stub_dir / "dnf", common + r'''
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log dnf "$@"
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[[ "${1:-}" == install && "${2:-}" == -y && "${3:-}" == git ]] || exit 97
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''')
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write_executable(stub_dir / "sshd", common + r'''
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log sshd "$@"
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exit 97
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''')
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write_executable(stub_dir / "systemctl", common + r'''
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log systemctl "$@"
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case "${1:-}:${2:-}" in
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reload:sshd|reload:ssh) exit 0 ;;
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*) exit 97 ;;
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esac
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''')
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for command in ("useradd", "usermod"):
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write_executable(stub_dir / command, common + f'''\nlog {command} "$@"\nexit 97\n''')
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def configure_case(name: str) -> tuple[Path, Path]:
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fixture_root = work / name / "root"
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stub_dir = work / name / "bin"
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calls = fixture_root / "calls"
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state = fixture_root / "state"
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ssh_dir = fixture_root / "home/gib/.ssh"
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root_ssh_dir = fixture_root / "root/.ssh"
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(fixture_root / "etc/ssh/sshd_config.d").mkdir(parents=True)
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ssh_dir.mkdir(parents=True)
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root_ssh_dir.mkdir(parents=True)
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stub_dir.mkdir(parents=True)
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state.mkdir()
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calls.touch()
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(state / "target-uid").write_text("1000\n")
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(state / "home").write_text("/home/gib\n")
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(state / "target-dir-meta").write_text("1000:700\n")
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(state / "target-key-meta").write_text("1000:600\n")
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(state / "root-key-meta").write_text("0:600\n")
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(fixture_root / "stub-install").write_text(
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"#!/usr/bin/env bash\nprintf 'install-handoff %s\\n' \"${PANAMA_PATH:-unset}\" >> \"$PANAMA_BOOT_FIXTURE_ROOT/calls\"\n"
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)
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(fixture_root / "stub-install").chmod(0o755)
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make_stubs(stub_dir, fixture_root, calls)
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target_keys = ssh_dir / "authorized_keys"
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root_keys = root_ssh_dir / "authorized_keys"
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if name == "missing":
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pass
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elif name == "empty":
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target_keys.touch()
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elif name == "comment-only":
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target_keys.write_text("# no usable key\n\n")
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elif name == "ssh-directory-symlink":
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shutil.rmtree(ssh_dir)
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alternate = fixture_root / "unsafe-ssh"
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alternate.mkdir()
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(fixture_root / "home/gib/.ssh").symlink_to(alternate)
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elif name == "authorized-keys-symlink":
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alternate = fixture_root / "unsafe-authorized-keys"
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alternate.write_text("ssh-ed25519 unsafe\n")
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target_keys.symlink_to(alternate)
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elif name == "directory-wrong-mode":
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target_keys.write_text("ssh-ed25519 target\n")
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(state / "target-dir-meta").write_text("1000:755\n")
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elif name == "root-copy-directory-wrong-mode":
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root_keys.write_text("ssh-ed25519 root\n")
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(state / "target-dir-meta").write_text("1000:755\n")
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elif name == "file-wrong-mode":
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target_keys.write_text("ssh-ed25519 target\n")
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(state / "target-key-meta").write_text("1000:644\n")
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elif name == "directory-wrong-owner":
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target_keys.write_text("ssh-ed25519 target\n")
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(state / "target-dir-meta").write_text("0:700\n")
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elif name == "file-wrong-owner":
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target_keys.write_text("ssh-ed25519 target\n")
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(state / "target-key-meta").write_text("0:600\n")
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elif name == "root-target-account":
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target_keys.write_text("ssh-ed25519 target\n")
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(state / "target-uid").write_text("0\n")
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elif name == "relative-home":
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target_keys.write_text("ssh-ed25519 target\n")
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(state / "home").write_text("home/gib\n")
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elif name == "safe-existing-key":
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target_keys.write_text("ssh-ed25519 target\n")
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elif name == "safe-root-key-copy":
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root_keys.write_text("ssh-ed25519 root\n")
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else:
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raise ValueError(name)
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return fixture_root, stub_dir
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def run_case(name: str) -> tuple[int, str, str, Path]:
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fixture_root, stub_dir = configure_case(name)
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master, slave = pty.openpty()
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def attach_terminal() -> None:
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fcntl.ioctl(0, termios.TIOCSCTTY, 0)
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env = {
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**os.environ,
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"PATH": f"{stub_dir}:/usr/bin:/bin",
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"PANAMA_BOOT_FIXTURE_ROOT": str(fixture_root),
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"PANAMA_PATH": f"{fixture_root}/home/gib/.local/share/Panama",
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"HOME": f"{fixture_root}/root",
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}
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process = subprocess.Popen(
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["bash", boot, "--server"],
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stdin=slave,
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stdout=slave,
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stderr=slave,
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env=env,
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start_new_session=True,
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preexec_fn=attach_terminal,
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)
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os.close(slave)
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os.write(master, b"gib\nY\n")
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chunks: list[bytes] = []
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while True:
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try:
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chunk = os.read(master, 4096)
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except OSError as error:
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if error.errno == errno.EIO:
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break
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raise
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if not chunk:
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break
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chunks.append(chunk)
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os.close(master)
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status = process.wait()
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calls = (fixture_root / "calls").read_text()
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output = b"".join(chunks).decode(errors="replace")
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return status, output, calls, fixture_root
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unsafe_cases = (
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"missing",
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"empty",
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"comment-only",
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"ssh-directory-symlink",
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"authorized-keys-symlink",
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"directory-wrong-mode",
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"root-copy-directory-wrong-mode",
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"file-wrong-mode",
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"directory-wrong-owner",
|
||||
"file-wrong-owner",
|
||||
"root-target-account",
|
||||
"relative-home",
|
||||
)
|
||||
for case in unsafe_cases:
|
||||
status, output, calls, fixture_root = run_case(case)
|
||||
if status != 0:
|
||||
note(f"{case}: bootstrap stopped with status {status}: {output.strip()}")
|
||||
if "SSH hardening unavailable" not in output:
|
||||
note(f"{case}: unsafe login path did not explain why hardening was unavailable")
|
||||
if "sshd -t" in calls:
|
||||
note(f"{case}: unsafe login path validated sshd")
|
||||
if "systemctl reload" in calls:
|
||||
note(f"{case}: unsafe login path reloaded SSH")
|
||||
if (fixture_root / "etc/ssh/sshd_config.d/90-panama.conf").exists():
|
||||
note(f"{case}: unsafe login path changed the SSH drop-in")
|
||||
if case == "root-copy-directory-wrong-mode" and (
|
||||
fixture_root / "home/gib/.ssh/authorized_keys"
|
||||
).exists():
|
||||
note("root-copy-directory-wrong-mode: copied a root key into an unsafe SSH directory")
|
||||
if "install-handoff " not in calls:
|
||||
note(f"{case}: unsafe login path did not hand off to install")
|
||||
|
||||
for case in ("safe-existing-key", "safe-root-key-copy"):
|
||||
status, output, calls, fixture_root = run_case(case)
|
||||
if status != 0:
|
||||
note(f"{case}: safe login path stopped with status {status}: {output.strip()}")
|
||||
if "SSH hardening unavailable" in output:
|
||||
note(f"{case}: safe login path was rejected")
|
||||
if "systemctl reload" not in calls:
|
||||
note(f"{case}: safe login path did not reach SSH hardening")
|
||||
if "install-handoff " not in calls:
|
||||
note(f"{case}: safe login path did not hand off to install")
|
||||
dropin = fixture_root / "etc/ssh/sshd_config.d/90-panama.conf"
|
||||
if (dropin.read_text() if dropin.exists() else "") != "PermitRootLogin no\nPasswordAuthentication no\n":
|
||||
note(f"{case}: safe login path did not write the expected SSH drop-in")
|
||||
if case == "safe-root-key-copy":
|
||||
keys = fixture_root / "home/gib/.ssh/authorized_keys"
|
||||
if not keys.exists() or keys.read_text() != "ssh-ed25519 root\n":
|
||||
note("safe-root-key-copy: root key was not copied to the target account")
|
||||
|
||||
if findings:
|
||||
print(f"root server bootstrap: {len(findings)} finding(s)", file=sys.stderr)
|
||||
for finding in findings:
|
||||
print(f" - {finding}", file=sys.stderr)
|
||||
raise SystemExit(1)
|
||||
|
||||
print("root server bootstrap: PASS")
|
||||
PY
|
||||
Reference in New Issue
Block a user