dotfiles/nixos/security.nix

{
  config,
  pkgs,
  lib,
  ...
}: {
  networking.firewall.checkReversePath = "loose";
  networking.firewall.enable = true;

  nix.settings.allowed-users = ["@users"];

  security.apparmor.enable = true;
  security.apparmor.killUnconfinedConfinables = true;
  security.forcePageTableIsolation = true;
  security.lockKernelModules = false;
  security.protectKernelImage = true;
  security.virtualisation.flushL1DataCache = "always";

  sops.age = {
    keyFile = "/var/lib/sops-nix/key.txt";
    generateKey = true;
  };

  boot.kernelParams = ["slub_debug=FZP" "page_poison=1" "page_alloc.shuffle=1"];

  boot.blacklistedKernelModules = [
    # Obscure network protocols
    "ax25"
    "netrom"
    "rose"

    # Old or rare or insufficiently audited filesystems
    "adfs"
    "affs"
    "bfs"
    "befs"
    "cramfs"
    "efs"
    "erofs"
    "exofs"
    "freevxfs"
    "f2fs"
    "hfs"
    "hpfs"
    "jfs"
    "minix"
    "nilfs2"
    "omfs"
    "qnx4"
    "qnx6"
    "sysv"
    "ufs"
  ];

  # Restrict ptrace() usage to processes with a pre-defined relationship
  # (e.g., parent/child)
  boot.kernel.sysctl."kernel.yama.ptrace_scope" = lib.mkOverride 500 1;

  # Hide kptrs even for processes with CAP_SYSLOG
  boot.kernel.sysctl."kernel.kptr_restrict" = lib.mkOverride 500 2;

  # Disable bpf() JIT (to eliminate spray attacks)
  boot.kernel.sysctl."net.core.bpf_jit_enable" = false;

  # Disable ftrace debugging
  boot.kernel.sysctl."kernel.ftrace_enabled" = false;

  # Enable strict reverse path filtering (that is, do not attempt to route
  # packets that "obviously" do not belong to the iface's network; dropped
  # packets are logged as martians).
  boot.kernel.sysctl."net.ipv4.conf.all.log_martians" = true;
  boot.kernel.sysctl."net.ipv4.conf.all.rp_filter" = "1";
  boot.kernel.sysctl."net.ipv4.conf.default.log_martians" = true;
  boot.kernel.sysctl."net.ipv4.conf.default.rp_filter" = "1";

  # Ignore broadcast ICMP (mitigate SMURF)
  boot.kernel.sysctl."net.ipv4.icmp_echo_ignore_broadcasts" = true;

  # Ignore incoming ICMP redirects (note: default is needed to ensure that the
  # setting is applied to interfaces added after the sysctls are set)
  boot.kernel.sysctl."net.ipv4.conf.all.accept_redirects" = false;
  boot.kernel.sysctl."net.ipv4.conf.all.secure_redirects" = false;
  boot.kernel.sysctl."net.ipv4.conf.default.accept_redirects" = false;
  boot.kernel.sysctl."net.ipv4.conf.default.secure_redirects" = false;
  boot.kernel.sysctl."net.ipv6.conf.all.accept_redirects" = false;
  boot.kernel.sysctl."net.ipv6.conf.default.accept_redirects" = false;

  # Ignore outgoing ICMP redirects (this is ipv4 only)
  boot.kernel.sysctl."net.ipv4.conf.all.send_redirects" = false;
  boot.kernel.sysctl."net.ipv4.conf.default.send_redirects" = false;
}