Sandboxing wrapper script for bubblewrap ; see https://sloonz.github.io/posts/sandboxing-3/

sandbox.py

#!/usr/bin/python

import argparse
import os
import shlex
import sys
import tempfile
import yaml

config = yaml.full_load(open(os.path.expanduser("~/.config/sandbox.yml")))
parser = argparse.ArgumentParser()
parser.add_argument("--name", "-n", action="store")
parser.add_argument("--preset", "-p", nargs=1, action="append")
parser.add_argument("--as", "-a", action="store")
bwrap_args0 = ("unshare-all", "share-net", "unshare-user", "unshare-user-try", "unshare-ipc", "unshare-net", "unshare-uts", "unshare-cgroup", "unshare-cgroup-try", "clearenv", "new-session", "die-with-parent", "as-pid-1")
bwrap_args1 = ("args", "userns", "userns2", "pidns", "uid", "gid", "hostname", "chdir", "unsetenv", "lock-file", "sync-fd", "remount-ro", "exec-label", "file-label", "proc", "dev", "tmpfs", "mqueue", "dir", "seccomp", "add-seccomp-fd", "block-fd", "userns-block-fd", "json-status-fd", "cap-add", "cap-drop", "perms")
bwrap_args2 = ("setenv", "bind", "bind-try", "dev-bind", "dev-bind-try", "ro-bind", "ro-bind-try", "file", "bind-data", "ro-bind-data", "symlink", "chmod")
for a in bwrap_args0:
    parser.add_argument("--" + a, action="store_true")
for a in bwrap_args1:
    parser.add_argument("--" + a, nargs=1, action="append")
for a in bwrap_args2:
    parser.add_argument("--" + a, nargs=2, action="append")
parser.add_argument("command", nargs="+")
args = parser.parse_args()

bwrap_command = ["bwrap"]
system_bus_args = set()
session_bus_args = set()

executable = getattr(args, "as") or args.command[0]
executable = executable.split("/")[-1]

def expand(s, extra_env):
    return str(s).format(env={**os.environ, **extra_env}, command=args.command, executable=executable, pid=os.getpid())

def handle_bind(params, create, typ, extra_env):
    if isinstance(params, str):
        params = [params, params]
    src, dst = params
    src = expand(src, extra_env)
    dst = expand(dst, extra_env)
    if create:
        os.makedirs(src, exist_ok=True)
    return ("--" + typ, src, dst)

def handle_setup(config, setup, extra_env):
    setup = setup.copy()
    setup_args = []

    use_params = setup.pop("use", None)
    if use_params:
        for preset in use_params:
            for preset_setup in config["presets"][preset]:
                setup_args.extend(handle_setup(config, preset_setup, extra_env))

    args_params = setup.pop("args", None)
    if args_params:
        setup_args.extend(expand(a, extra_env) for a in args_params)

    setenv_params = setup.pop("setenv", None)
    if isinstance(setenv_params, dict):
        for k, v in setenv_params.items():
            extra_env[k] = expand(v, extra_env)
            setup_args.extend(("--setenv", k, extra_env[k]))
    elif isinstance(setenv_params, list):
        for k in setenv_params:
            if k in os.environ:
                setup_args.extend(("--setenv", k, os.environ[k]))

    for bind_type in ("bind", "bind-try", "dev-bind", "dev-bind-try", "ro-bind", "ro-bind-try"):
        bind_params = setup.pop(bind_type, None)
        if bind_params:
            setup_args.extend(handle_bind(bind_params, setup.pop("bind-create", None), bind_type, extra_env))

    for dbus_setup in ("see", "talk", "own", "call", "broadcast"):
        dbus_setup_params = setup.pop("dbus-" + dbus_setup, None)
        if dbus_setup_params:
            is_system = setup.pop("system-bus", False)
            if is_system:
                system_bus_args.add("--%s=%s" % (dbus_setup, dbus_setup_params))
            else:
                session_bus_args.add("--%s=%s" % (dbus_setup, dbus_setup_params))

    file_params = setup.pop("file", None)
    if file_params:
        data, dst = file_params
        pr, pw = os.pipe2(0)
        if os.fork() == 0:
            os.close(pr)
            os.write(pw, data.encode())
            sys.exit(0)
        else:
            os.close(pw)
            setup_args.extend(("--file", str(pr), expand(dst, extra_env)))

    dir_params = setup.pop("dir", None)
    if dir_params:
        setup_args.extend(("--dir", expand(dir_params, extra_env)))

    bind_args_params = setup.pop("bind-args", None)
    if bind_args_params:
        added_paths = set()
        strict = setup.pop("strict", True)
        ro = setup.pop("ro", True)
        for a in args.command[1:]:
            if os.path.exists(a):
                path = os.path.abspath(a)
                if not strict:
                    path = os.path.dirname(path)
                if not path in added_paths:
                    setup_args.extend((ro and "--ro-bind" or "--bind", path, path))
                    added_paths.add(path)

    cwd = os.getcwd()

    bind_cwd_params = setup.pop("bind-cwd", None)
    if bind_cwd_params is not None:
        ro = setup.pop("ro", False)
        setup_args.extend((ro and "--ro-bind" or "--bind", cwd, cwd))

    cwd_params = setup.pop("cwd", None)
    if cwd_params is not None:
        if type(cwd_params) == "str":
            setup_args.extend(("--chdir", expand(cwd_params, extra_env)))
        elif cwd_params:
            setup_args.extend(("--chdir", cwd))

    if setup.pop("restrict-tty", None):
        # --new-session breaks interactive sessions, this is an alternative way of fixing CVE-2017-5226
        import seccomp
        import termios
        f = seccomp.SyscallFilter(defaction=seccomp.ALLOW)
        f.add_rule(seccomp.KILL_PROCESS, "ioctl", seccomp.Arg(1, seccomp.MASKED_EQ, 0xffffffff, termios.TIOCSTI))
        f.add_rule(seccomp.KILL_PROCESS, "ioctl", seccomp.Arg(1, seccomp.MASKED_EQ, 0xffffffff, termios.TIOCLINUX))
        f.load()

    if len(setup) != 0:
        print("unknown setup actions: %s" % list(setup.keys()))
        sys.exit(1)

    return setup_args

def exec_bwrap(rule):
    extra_env = {}
    for setup in rule.get("setup", []):
        bwrap_command.extend(handle_setup(config, setup, extra_env))
    for (preset,) in args.preset or []:
        for preset_setup in config["presets"][preset]:
            bwrap_command.extend(handle_setup(config, preset_setup, extra_env))
    for a in bwrap_args0:
        if getattr(args, a.replace("-", "_")):
            bwrap_command.append("--" + a)
    for a in bwrap_args1:
        for (val,) in getattr(args, a.replace("-", "_")) or []:
            bwrap_command.extend(("--" + a, val))
    for a in bwrap_args2:
        for (v1, v2) in getattr(args, a.replace("-", "_")) or []:
            bwrap_command.extend(("--" + a, v1, v2))
    
    dbus_proxy_args = []
    dbus_proxy_dir = f"{os.environ['XDG_RUNTIME_DIR']}/xdg-dbus-proxy"
    if session_bus_args or system_bus_args:
        os.makedirs(dbus_proxy_dir, exist_ok=True)
    if session_bus_args:
        proxy_socket = tempfile.mktemp(prefix="session-", dir=dbus_proxy_dir)
        dbus_proxy_args.extend((os.environ["DBUS_SESSION_BUS_ADDRESS"], proxy_socket))
        dbus_proxy_args.append("--filter")
        dbus_proxy_args.extend(session_bus_args)
        bwrap_command.extend(("--bind", proxy_socket, os.environ["DBUS_SESSION_BUS_ADDRESS"].removeprefix("unix:path="),
            "--setenv", "DBUS_SESSION_BUS_ADDRESS", os.environ["DBUS_SESSION_BUS_ADDRESS"]))
    if system_bus_args:
        proxy_socket = tempfile.mktemp(prefix="system-", dir=dbus_proxy_dir)
        dbus_proxy_args.extend(("/run/dbus/system_bus_socket", proxy_socket))
        dbus_proxy_args.append("--filter")
        dbus_proxy_args.extend(system_bus_args)
        bwrap_command.extend(("--bind", "/run/dbus/system_bus_socket", "/run/dbus/system_bus_socket"))
    if dbus_proxy_args:
        pr, pw = os.pipe2(0)
        if os.fork() == 0:
            os.close(pr)
            dbus_proxy_command = ["xdg-dbus-proxy", "--fd=%d" % pw] + list(dbus_proxy_args)
            os.execlp(dbus_proxy_command[0], *dbus_proxy_command)
        # I would like to use bwrap's --block-fd, but bwrap setups then wait, and therefore may try to bind an non-existent socket
        assert os.read(pr, 1) == b"x" # wait for xdg-dbus-proxy to be ready
        bwrap_command.extend(("--sync-fd", str(pr)))

    bwrap_command.extend(args.command)
    if os.getenv("SANDBOX_DEBUG") == "1":
        print(bwrap_command, file=sys.stderr)
    os.execvp(bwrap_command[0], bwrap_command)

for rule in config["rules"]:
    is_match = False
    assert not (set(rule.keys()) - {"match", "no-sandbox", "setup"})
    if "match" in rule:
        assert not (set(rule["match"].keys()) - {"bin", "name"})
        if executable and rule["match"].get("bin") == executable:
            is_match = True
        if args.name and rule["match"].get("name") == args.name:
            is_match = True
    else:
        is_match = True

    if is_match:
        if rule.get("no-sandbox"):
            os.execvp(args.command[0], args.command)
        else:
            exec_bwrap(rule)
        break

sandbox.yml

presets:
  common:
    - args: [--clearenv, --unshare-pid, --die-with-parent, --proc, /proc, --dev, /dev, --tmpfs, /tmp, --new-session]
    - setenv: [PATH, LANG, XDG_RUNTIME_DIR, XDG_SESSION_TYPE, TERM, HOME, LOGNAME, USER]
    - ro-bind: /etc
    - ro-bind: /usr
    - args: [--symlink, usr/bin, /bin, --symlink, usr/bin, /sbin, --symlink, usr/lib, /lib, --symlink, usr/lib, /lib64, --tmpfs, "{env[XDG_RUNTIME_DIR]}"]
    - bind: /run/systemd/resolve
  private-home:
    - bind: ["{env[HOME]}/sandboxes/{executable}/", "{env[HOME]}"]
      bind-create: true
    - dir: "{env[HOME]}/.config"
    - dir: "{env[HOME]}/.cache"
    - dir: "{env[HOME]}/.local/share"
  x11:
    - setenv: [DISPLAY]
    - ro-bind: /tmp/.X11-unix/
  wayland:
    - setenv: [WAYLAND_DISPLAY]
    - ro-bind: "{env[XDG_RUNTIME_DIR]}/{env[WAYLAND_DISPLAY]}"
  pulseaudio:
    - ro-bind: "{env[XDG_RUNTIME_DIR]}/pulse/native"
    - ro-bind-try: "{env[HOME]}/.config/pulse/cookie"
    - ro-bind-try: "{env[XDG_RUNTIME_DIR]}/pipewire-0"
  drm:
    - dev-bind: /dev/dri
    - ro-bind: /sys
  portal:
    - file: ["", "{env[XDG_RUNTIME_DIR]}/flatpak-info"]
    - file: ["", "/.flatpak-info"]
    - dbus-call: "org.freedesktop.portal.*=*"
    - dbus-broadcast: "org.freedesktop.portal.*=@/org/freedesktop/portal/*"
rules:
  - match:
      bin: firefox
    setup:
      - setenv:
          MOZ_ENABLE_WAYLAND: 1
      - use: [common, private-home, wayland, portal]
      - dbus-own: org.mozilla.firefox.*
      - bind: "{env[HOME]}/Downloads"
      - bind: ["{env[HOME]}/.config/mozilla", "{env[HOME]}/.mozilla"]

  - match:
      name: shell
    setup:
      - use: [common, private-home]

  - match:
      bin: node
    setup:
      - use: [common, private-home]
      - bind-cwd: {}
      - cwd: true

  - match:
      bin: npx
    setup:
      - use: [common, private-home]
      - bind-cwd: {}
      - cwd: true

  - match:
      bin: npm
    setup:
      - use: [common, private-home]
      - bind-cwd: {}
      - cwd: true
  - match:
      name: none

  # Fallback: anything else fall backs to a sandboxed empty home
  - setup:
    - use: [common, private-home, x11, wayland, pulseaudio, portal]

I have a yubikey that I'd like to use with firefox. I can give access to all of /dev to make it work but would like to avoid that and only what's necessary. However, I'm not entirely sure what files that are and how to add them. At the very least two /dev/hidraw* devices get added when plugging the key in.

I thought I could do something with udev rules and symlinks, but the link doesn't point anywhere in the sandbox since I don't add the hidraw device itself. As the devices get created only after you plug in the key I can't use dev-bind-try since they don't get added retroactively.

Is dev-binding /dev my only option here (how flatpak does it)?