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security: simplify and fix dbus configuration

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Previously, due to the dependency on a helper service, this dbus config
was initially inactive after system boot, allowing for unrestricted use
of the problematic dbus call.
This also broke the accompanying VM test on faster systems.

Remove 'allow' policy for root because it's a no-op:
1. It's overridden by the 'mandatory' deny policy.
2. Root can use all dbus calls anyways, regardless of policy settings.

Also, add some comments.
This commit is contained in:
Erik Arvstedt 2020-08-20 13:11:07 +02:00
parent 343e026030
commit 96ea2e671c
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GPG Key ID: 33312B944DD97846
1 changed files with 11 additions and 39 deletions
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50
modules/security.nix
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@ -1,55 +1,27 @@
{ config, lib, pkgs, ... }:
with lib;
{
# Only show the current user's processes in /proc.
# Users with group 'proc' can still access all processes.
security.hideProcessInformation = true;
let
inherit (config) nix-bitcoin-services;
dataDir = "/var/lib/dbus-hardening";
# Mitigates a security issue that allows unprivileged users to read
# other unprivileged user's processes' credentials from CGroup using
# `systemctl status`.
dbus-hardening = pkgs.writeText "dbus.conf" ''
# This mitigates a systemd security issue leaking (sub)process
# command lines.
# Only allow root to retrieve systemd unit information like
# cgroup paths (i.e. (sub)process command lines) via D-Bus.
# This D-Bus call is used by `systemctl status`.
services.dbus.packages = [ (pkgs.writeTextDir "etc/dbus-1/system.d/dbus.conf" ''
<?xml version="1.0" encoding="UTF-8"?> <!-- -*- XML -*- -->
<!DOCTYPE busconfig PUBLIC
"-//freedesktop//DTD D-BUS Bus Configuration 1.0//EN"
"http://www.freedesktop.org/standards/dbus/1.0/busconfig.dtd">
<busconfig>
<policy user="root">
<allow send_destination="org.freedesktop.systemd1"
send_interface="org.freedesktop.systemd1.Manager"
send_member="GetUnitProcesses"/>
</policy>
<policy context="mandatory">
<deny send_destination="org.freedesktop.systemd1"
send_interface="org.freedesktop.systemd1.Manager"
send_member="GetUnitProcesses"/>
</policy>
</busconfig>
'';
in {
config = {
systemd.tmpfiles.rules = [
"d '${dataDir}/etc/dbus-1/system.d' 0770 messagebus messagebus - -"
];
services.dbus.packages = [ "${dataDir}" ];
systemd.services.hardeneddbus = {
description = "Install hardeneddbus";
wantedBy = [ "multi-user.target" ];
script = ''
cp ${dbus-hardening} ${dataDir}/etc/dbus-1/system.d/dbus.conf
chmod 640 ${dataDir}/etc/dbus-1/system.d/dbus.conf
'';
serviceConfig = nix-bitcoin-services.defaultHardening // {
PrivateNetwork = "true";
Type = "oneshot";
User = "messagebus";
ReadWritePaths = "${dataDir}";
};
};
};
'') ];
}