diff options
Diffstat (limited to 'nixos/modules/security/systemd-confinement.nix')
| -rw-r--r-- | nixos/modules/security/systemd-confinement.nix | 202 |
1 files changed, 202 insertions, 0 deletions
diff --git a/nixos/modules/security/systemd-confinement.nix b/nixos/modules/security/systemd-confinement.nix new file mode 100644 index 00000000000..f3a2de3bf87 --- /dev/null +++ b/nixos/modules/security/systemd-confinement.nix @@ -0,0 +1,202 @@ +{ config, pkgs, lib, utils, ... }: + +let + toplevelConfig = config; + inherit (lib) types; + inherit (utils.systemdUtils.lib) mkPathSafeName; +in { + options.systemd.services = lib.mkOption { + type = types.attrsOf (types.submodule ({ name, config, ... }: { + options.confinement.enable = lib.mkOption { + type = types.bool; + default = false; + description = '' + If set, all the required runtime store paths for this service are + bind-mounted into a <literal>tmpfs</literal>-based <citerefentry> + <refentrytitle>chroot</refentrytitle> + <manvolnum>2</manvolnum> + </citerefentry>. + ''; + }; + + options.confinement.fullUnit = lib.mkOption { + type = types.bool; + default = false; + description = '' + Whether to include the full closure of the systemd unit file into the + chroot, instead of just the dependencies for the executables. + + <warning><para>While it may be tempting to just enable this option to + make things work quickly, please be aware that this might add paths + to the closure of the chroot that you didn't anticipate. It's better + to use <option>confinement.packages</option> to <emphasis + role="strong">explicitly</emphasis> add additional store paths to the + chroot.</para></warning> + ''; + }; + + options.confinement.packages = lib.mkOption { + type = types.listOf (types.either types.str types.package); + default = []; + description = let + mkScOption = optName: "<option>serviceConfig.${optName}</option>"; + in '' + Additional packages or strings with context to add to the closure of + the chroot. By default, this includes all the packages from the + ${lib.concatMapStringsSep ", " mkScOption [ + "ExecReload" "ExecStartPost" "ExecStartPre" "ExecStop" + "ExecStopPost" + ]} and ${mkScOption "ExecStart"} options. If you want to have all the + dependencies of this systemd unit, you can use + <option>confinement.fullUnit</option>. + + <note><para>The store paths listed in <option>path</option> are + <emphasis role="strong">not</emphasis> included in the closure as + well as paths from other options except those listed + above.</para></note> + ''; + }; + + options.confinement.binSh = lib.mkOption { + type = types.nullOr types.path; + default = toplevelConfig.environment.binsh; + defaultText = lib.literalExpression "config.environment.binsh"; + example = lib.literalExpression ''"''${pkgs.dash}/bin/dash"''; + description = '' + The program to make available as <filename>/bin/sh</filename> inside + the chroot. If this is set to <literal>null</literal>, no + <filename>/bin/sh</filename> is provided at all. + + This is useful for some applications, which for example use the + <citerefentry> + <refentrytitle>system</refentrytitle> + <manvolnum>3</manvolnum> + </citerefentry> library function to execute commands. + ''; + }; + + options.confinement.mode = lib.mkOption { + type = types.enum [ "full-apivfs" "chroot-only" ]; + default = "full-apivfs"; + description = '' + The value <literal>full-apivfs</literal> (the default) sets up + private <filename class="directory">/dev</filename>, <filename + class="directory">/proc</filename>, <filename + class="directory">/sys</filename> and <filename + class="directory">/tmp</filename> file systems in a separate user + name space. + + If this is set to <literal>chroot-only</literal>, only the file + system name space is set up along with the call to <citerefentry> + <refentrytitle>chroot</refentrytitle> + <manvolnum>2</manvolnum> + </citerefentry>. + + <note><para>This doesn't cover network namespaces and is solely for + file system level isolation.</para></note> + ''; + }; + + config = let + rootName = "${mkPathSafeName name}-chroot"; + inherit (config.confinement) binSh fullUnit; + wantsAPIVFS = lib.mkDefault (config.confinement.mode == "full-apivfs"); + in lib.mkIf config.confinement.enable { + serviceConfig = { + RootDirectory = "/var/empty"; + TemporaryFileSystem = "/"; + PrivateMounts = lib.mkDefault true; + + # https://github.com/NixOS/nixpkgs/issues/14645 is a future attempt + # to change some of these to default to true. + # + # If we run in chroot-only mode, having something like PrivateDevices + # set to true by default will mount /dev within the chroot, whereas + # with "chroot-only" it's expected that there are no /dev, /proc and + # /sys file systems available. + # + # However, if this suddenly becomes true, the attack surface will + # increase, so let's explicitly set these options to true/false + # depending on the mode. + MountAPIVFS = wantsAPIVFS; + PrivateDevices = wantsAPIVFS; + PrivateTmp = wantsAPIVFS; + PrivateUsers = wantsAPIVFS; + ProtectControlGroups = wantsAPIVFS; + ProtectKernelModules = wantsAPIVFS; + ProtectKernelTunables = wantsAPIVFS; + }; + confinement.packages = let + execOpts = [ + "ExecReload" "ExecStart" "ExecStartPost" "ExecStartPre" "ExecStop" + "ExecStopPost" + ]; + execPkgs = lib.concatMap (opt: let + isSet = config.serviceConfig ? ${opt}; + in lib.flatten (lib.optional isSet config.serviceConfig.${opt})) execOpts; + unitAttrs = toplevelConfig.systemd.units."${name}.service"; + allPkgs = lib.singleton (builtins.toJSON unitAttrs); + unitPkgs = if fullUnit then allPkgs else execPkgs; + in unitPkgs ++ lib.optional (binSh != null) binSh; + }; + })); + }; + + config.assertions = lib.concatLists (lib.mapAttrsToList (name: cfg: let + whatOpt = optName: "The 'serviceConfig' option '${optName}' for" + + " service '${name}' is enabled in conjunction with" + + " 'confinement.enable'"; + in lib.optionals cfg.confinement.enable [ + { assertion = !cfg.serviceConfig.RootDirectoryStartOnly or false; + message = "${whatOpt "RootDirectoryStartOnly"}, but right now systemd" + + " doesn't support restricting bind-mounts to 'ExecStart'." + + " Please either define a separate service or find a way to run" + + " commands other than ExecStart within the chroot."; + } + { assertion = !cfg.serviceConfig.DynamicUser or false; + message = "${whatOpt "DynamicUser"}. Please create a dedicated user via" + + " the 'users.users' option instead as this combination is" + + " currently not supported."; + } + { assertion = cfg.serviceConfig ? ProtectSystem -> cfg.serviceConfig.ProtectSystem == false; + message = "${whatOpt "ProtectSystem"}. ProtectSystem is not compatible" + + " with service confinement as it fails to remount /usr within" + + " our chroot. Please disable the option."; + } + ]) config.systemd.services); + + config.systemd.packages = lib.concatLists (lib.mapAttrsToList (name: cfg: let + rootPaths = let + contents = lib.concatStringsSep "\n" cfg.confinement.packages; + in pkgs.writeText "${mkPathSafeName name}-string-contexts.txt" contents; + + chrootPaths = pkgs.runCommand "${mkPathSafeName name}-chroot-paths" { + closureInfo = pkgs.closureInfo { inherit rootPaths; }; + serviceName = "${name}.service"; + excludedPath = rootPaths; + } '' + mkdir -p "$out/lib/systemd/system/$serviceName.d" + serviceFile="$out/lib/systemd/system/$serviceName.d/confinement.conf" + + echo '[Service]' > "$serviceFile" + + # /bin/sh is special here, because the option value could contain a + # symlink and we need to properly resolve it. + ${lib.optionalString (cfg.confinement.binSh != null) '' + binsh=${lib.escapeShellArg cfg.confinement.binSh} + realprog="$(readlink -e "$binsh")" + echo "BindReadOnlyPaths=$realprog:/bin/sh" >> "$serviceFile" + ''} + + while read storePath; do + if [ -L "$storePath" ]; then + # Currently, systemd can't cope with symlinks in Bind(ReadOnly)Paths, + # so let's just bind-mount the target to that location. + echo "BindReadOnlyPaths=$(readlink -e "$storePath"):$storePath" + elif [ "$storePath" != "$excludedPath" ]; then + echo "BindReadOnlyPaths=$storePath" + fi + done < "$closureInfo/store-paths" >> "$serviceFile" + ''; + in lib.optional cfg.confinement.enable chrootPaths) config.systemd.services); +} |