# Linux Hardening

> Work in progress.

Application sandboxing (nixpak/bubblewrap) and Linux hardening knowledge for this repository. For
the repository security policy, architecture and audit status, see [`SECURITY.md`](../SECURITY.md);
for temporary exceptions and known gaps, see [`WORKAROUNDS.md`](../WORKAROUNDS.md).

## Goal

- **System Level**: Protect critical files from being accessed by untrusted applications.
  1. Such as browser cookies, SSH keys, etc.
- **Per-App Level**: Prevent untrusted applications(such as closed-source apps) from:
  1. Accessing files they shouldn't.
     - Such as a malicious application accessing your browser's cookies, SSH keys, etc.
  1. Accessing the network when they don't need to.
  1. Accessing hardware devices they don't need.

## Current Structure

### 1. **System Level**

- **AppArmor** (`apparmor/`): AppArmor profiles and configuration
- **Kernel & process hardening** (`kernel-hardening.nix`): sysctl baseline and unused-module deny
  list; rationale and verification live in [`SECURITY.md`](../SECURITY.md).

### 2. **Per-App Level**

- **Nixpak** (`nixpaks/`): Bubblewrap-based sandboxing for applications
  - Firefox configuration
  - QQ (Chinese messaging app) configuration
  - Telegram Desktop configuration
  - Modular system with reusable components
- **Bubblewrap** (`bwraps/`): Direct bubblewrap configurations
  - WeChat sandboxing configuration

## Current Implementation Status

| Component         | Status    | Notes                         |
| ----------------- | --------- | ----------------------------- |
| AppArmor Profiles | 🚧 WIP    | Basic structure in place      |
| Nixpak Firefox    | ✅ Active | Firefox sandboxing via nixpak |
| Nixpak QQ         | ✅ Active | QQ application sandboxing     |
| Nixpak Telegram   | ✅ Active | Telegram Desktop sandboxing   |
| Bubblewrap WeChat | ✅ Active | WeChat specific sandboxing    |

## Directory Structure

```
hardening/
├── README.md
├── kernel-hardening.nix # sysctl baseline + unused-module deny list
├── apparmor/           # AppArmor security profiles
│   └── default.nix
├── bwraps/            # Direct bubblewrap configurations
│   ├── default.nix
│   └── wechat.nix
└── nixpaks/           # Nixpak application sandboxing
    ├── default.nix
    ├── firefox.nix
    ├── qq.nix
    ├── telegram-desktop.nix
    └── modules/       # Reusable nixpak modules
        ├── custom-policy.nix
        ├── gui-base.nix
        └── network.nix
```

## System Hardening

- Apparmor: [roddhjav/apparmor.d](https://github.com/roddhjav/apparmor.d)
  - https://gitlab.com/apparmor/apparmor/-/wikis/Documentation
  - AppArmor.d is a set of over 1500 AppArmor profiles whose aim is to confine most Linux based
    applications and processes.
  - But its profiles assume an FHS filesystem, so most of them never match the Nix store paths a
    NixOS system actually executes.
  - Apparmor on NixOS Roadmap:
    - https://discourse.nixos.org/t/apparmor-on-nixos-roadmap/57217
    - https://github.com/LordGrimmauld/aa-alias-manager
- SELinux: too complex, not recommended for personal use.

## Application Sandboxing

- [Bubblewrap](https://github.com/containers/bubblewrap):
  [nixpak](https://github.com/nixpak/nixpak), more secure than firejail, but no batteries included.
  - NixOS's FHSEnv is implemented using bubblewrap by default.
- [Firejail](https://github.com/netblue30/firejail/tree/master/etc): A SUID security sandbox with
  hundreds of security profiles for many common applications in the default installation.
  - https://wiki.nixos.org/wiki/Firejail
  - Firejail needs SUID to work, which is considered a security risk -
    [Does firejail improve the security of my system?](https://github.com/netblue30/firejail/discussions/4601)
- [Systemd/Hardening](https://wiki.nixos.org/wiki/Systemd/Hardening): Systemd also provides some
  sandboxing features.

## NOTE

**Running untrusted code is never safe, kernel hardening & sandboxing cannot change this**; run
untrusted code in a VM on an isolated network. The threat model lives in
[`SECURITY.md`](../SECURITY.md).

## References

- [Harden your NixOS workstation - dataswamp](https://dataswamp.org/~solene/2022-01-13-nixos-hardened.html)
- [Linux Insecurities - Madaidans](https://madaidans-insecurities.github.io/linux.html)
- [Sandboxing all programs by default - NixOS Discourse](https://discourse.nixos.org/t/sandboxing-all-programs-by-default/7792)
- [Paranoid NixOS Setup - xeiaso](https://xeiaso.net/blog/paranoid-nixos-2021-07-18/)
- [nix-mineral](https://github.com/cynicsketch/nix-mineral): NixOS module for convenient system
  hardening.
- apparmor configs:
  - https://github.com/zramctl/dotfiles/blob/4fe177f6984154960942bb47d5a375098ec6ed6a/modules/nixos/security/apparmor.nix#L4
  - https://git.grimmauld.de/Grimmauld/grimm-nixos-laptop/src/branch/main/hardening