Title: Harden your NixOS workstation
Author: Solène
Date: 13 January 2022
Tags: nix nixos security
Description: This explains how to tweak your NixOS system to make it
hardened in regards to security
# Introduction
Coming from an OpenBSD background, I wanted to harden my NixOS system
for better security. As you may know (or not), security mitigations
must be thought against a security threat model. My model here is to
prevent web browsers to leak data, prevent services to be exploitable
remotely and prevent programs from being exploited to run malicious
code.
NixOS comes with a few settings to improve in these areas, I'll share a
sample of configuration to increase the default security. Unrelated to
security defense itself, but you should absolutely encrypt your
filesystem, so in case of physical access to your computer no data
could be extracted.
# Use the hardened profile
There are a few profiles available by default in NixOS which are files
with a set of definitions and one of them is named "hardened" because
it enables many security measures.
(HTM) Link to the hardened profile definition
Here is a simplified list of important changes:
* use the hardened Linux kernel (different defaults and some extra
patches from https://github.com/anthraxx/linux-hardened/)
* use the memory allocator "scudo", protecting against some buffer
overflow exploits
* prevent kernel modules to be loaded after boot
* protect against rewriting kernel image
* increase containers/virtualization protection at a performance cost
(L1 flush or page table isolation)
* apparmor is enabled by default
* many filesystem modules are forbidden because old/rare/not audited
enough
* many other specific tweaks
Of course, using this mode will slightly reduce the system performance
and may trigger some runtime problems due to the memory management
being less permissive. On one hand, it's good because it allows to
catch programming errors, but on the other hand it's not fun to have
your programs crashing when you need them.
With the scudo memory allocator, I have troubles running Firefox, it
will only start after 2 or 3 crashes and then will work fine. There is
a less permissive allocator named graphene-hardened, but I had too much
troubles running programs with it.
# Use firewall
One simple rule is to block any incoming traffic that would connect to
listening services. It's way more secure to block everything and then
allow the services you know must be open to the outside than relying on
the service's configuration to not listen on public interfaces.
# Use Clamav
Clamav is an antivirus, and yes it can be useful on Linux. If it can
prevent you at least once to run a hostile binary, then it's worth
running it.
# Firejail
I featured firejail previously on my blog, I'm convinced of its
usefulnes. You can run a program using firejail, and it will restrict
its permissions and rights so in case of security breach, the program
will be restricted.
This is rather important to run web browsers with it because it will
prevent them any access to the filesystem except ~/Downloads/ and a few
required directories (local profile, /etc/resolv.conf, font cache
etc...).
# Enable this on NixOS
Because NixOS is declarative, it's easy to share the configuration. My
configuration supports both Firefox and Chromium, you can remove the
related lines you don't need.
Be careful about the import declaration, you certainly already have one
for the ./hardware-configuration.nix file.
```/etc/nixos/configuration.nix file
imports =
[
./hardware-configuration.nix
<nixpkgs/nixos/modules/profiles/hardened.nix>
];
# enable firewall and block all ports
networking.firewall.enable = true;
networking.firewall.allowedTCPPorts = [];
networking.firewall.allowedUDPPorts = [];
# disable coredump that could be exploited later
# and also slow down the system when something crash
systemd.coredump.enable = false;
# required to run chromium
security.chromiumSuidSandbox.enable = true;
# enable firejail
programs.firejail.enable = true;
# create system-wide executables firefox and chromium
# that will wrap the real binaries so everything
# work out of the box.
programs.firejail.wrappedBinaries = {
firefox = {
executable = "${pkgs.lib.getBin pkgs.firefox}/bin/firefox";
profile = "${pkgs.firejail}/etc/firejail/firefox.profile";
};
chromium = {
executable = "${pkgs.lib.getBin pkgs.chromium}/bin/chromium";
profile = "${pkgs.firejail}/etc/firejail/chromium.profile";
};
};
# enable antivirus clamav and
# keep the signatures' database updated
services.clamav.daemon.enable = true;
services.clamav.updater.enable = true;
```
Rebuild the system, reboot and enjoy your new secure system.
# Going further: network filtering
If you want to absolutely control your network connections, I'd
absolutely recommend the service OpenSnitch. This is a daemon that
will listen to all the network done on the system and allow you to
allow/block connections per executable/source/destination/protocol/many
parameters.
OpenSnitch comes with a GUI app called opensnitch-ui which is
mandatory, if the ui is not running, no filtering is done. When the ui
is running, every time a new connection is not matching an existing
rule, you will be prompted with information telling you what executable
is trying to do on which protocol with which host, then you can decide
how long you allow this (or block).
Just use `services.opensnitch.enable = true;` in the system
configuration and run opensnitch-ui program in your graphical session.
To have persistent rules, open opensnitch-ui, go in the Preferences
menu and tab Database, choose "Database type: File" and pick a path to
save it (it's a sqlite database).
From this point, you will have to allow / block all network done on
your system, it can be time-consuming at first, but it's user-friendly
enough and rules can be done like "allow this entire executable" so you
don't have to allow every website visited by your web browser (but you
could!). You may be surprised by the amount of traffic done by non
networking programs. After some time, the rule set should be able to
cope with most of your needs without needing to add new entries.
(HTM) OpenSnitch wiki: getting started