fix(k3s): deny credential-bearing host ports to pod sources (#399)

* fix(k3s): deny credential-bearing host ports to pod sources

* test: assert the k3s pod firewall drops sensitive ports

* docs(security): record the pod firewall boundary

SECURITY.md

@@ -198,6 +198,11 @@ checks remain necessary after deployment.
   `sec=sys,no_root_squash`. File mode 0755 does not contain an allowed client with root privileges;
   such a client can act as server root within this export. The network allowlist and client trust
   are therefore material boundaries, not merely filesystem permissions.
+- **Pod-to-host firewall:** pod CIDR sources (`10.0.0.0/8`, `fd00::/104`) are accepted on the k3s
+  nodes, but the credential-bearing host ports are denied first (SSH, NFS, Postgres, restic, VNC,
+  Immich, and the exporter/monitoring ports). The broad accept stays so Cilium and in-cluster
+  service traffic keep working; a full port allowlist is future work. Ports 80/443 are deliberately
+  not denied, so in-cluster calls through the ingress are not broken.
 
 Only selected pod security fields, RBAC rules/binding metadata, listeners and export metadata were
 queried. Secret contents and raw kubeconfigs were not inspected. No live access policy, workload,

lib/genK3sAgentModule.nix

@@ -25,10 +25,16 @@ in
     enableIPv6 = true;
   };
 
-  # Cilium pods reach host services (e.g. the API server) from the pod CIDR.
+  # Pods may reach the host services they need, but they are not part of the
-  # `mkAfter` keeps this after the shared base rules (incl. the node_exporter
+  # host's trusted LAN. Deny the credential-bearing host ports before the broad
-  # drop), so pods cannot scrape node_exporter either.
+  # pod-network accept below, so a compromised pod cannot reach NFS, the
+  # database, the backup server, the VNC consoles, or an exporter. `mkAfter`
+  # keeps this after the shared base rules (incl. the node_exporter drop).
+  # Ports 80/443 stay reachable so in-cluster calls through the ingress are not
+  # broken; the ingress is the authentication boundary there.
   networking.firewall.extraInputRules = pkgs.lib.mkAfter ''
+    ip  saddr 10.0.0.0/8 tcp dport { 22, 2049, 2283, 5432, 5900, 5901, 5902, 5903, 8000, 8081, 8082, 9090, 9093, 9100, 9633, 9835 } drop
+    ip6 saddr fd00::/104 tcp dport { 22, 2049, 2283, 5432, 5900, 5901, 5902, 5903, 8000, 8081, 8082, 9090, 9093, 9100, 9633, 9835 } drop
     ip  saddr 10.0.0.0/8 accept
     ip6 saddr fd00::/104 accept
   '';

lib/genK3sServerModule.nix

@@ -52,10 +52,16 @@ in
     enableIPv6 = true;
   };
 
-  # Cilium pods reach host services (e.g. the API server) from the pod CIDR.
+  # Pods may reach the host services they need, but they are not part of the
-  # `mkAfter` keeps this after the shared base rules (incl. the node_exporter
+  # host's trusted LAN. Deny the credential-bearing host ports before the broad
-  # drop), so pods cannot scrape node_exporter either.
+  # pod-network accept below, so a compromised pod cannot reach NFS, the
+  # database, the backup server, the VNC consoles, or an exporter. `mkAfter`
+  # keeps this after the shared base rules (incl. the node_exporter drop).
+  # Ports 80/443 stay reachable so in-cluster calls through the ingress are not
+  # broken; the ingress is the authentication boundary there.
   networking.firewall.extraInputRules = lib.mkAfter ''
+    ip  saddr 10.0.0.0/8 tcp dport { 22, 2049, 2283, 5432, 5900, 5901, 5902, 5903, 8000, 8081, 8082, 9090, 9093, 9100, 9633, 9835 } drop
+    ip6 saddr fd00::/104 tcp dport { 22, 2049, 2283, 5432, 5900, 5901, 5902, 5903, 8000, 8081, 8082, 9090, 9093, 9100, 9633, 9835 } drop
     ip  saddr 10.0.0.0/8 accept
     ip6 saddr fd00::/104 accept
   '';

outputs/x86_64-linux/tests/security-k3s-pod-firewall/expected.nix

@@ -0,0 +1,5 @@
+{
+  lib,
+  outputs,
+}:
+lib.genAttrs (builtins.attrNames outputs.nixosConfigurations) (_: true)

outputs/x86_64-linux/tests/security-k3s-pod-firewall/expr.nix

@@ -0,0 +1,33 @@
+{
+  lib,
+  outputs,
+}:
+let
+  # Index of the first occurrence of `needle` in `haystack`, or null.
+  firstIndex =
+    needle: haystack:
+    let
+      parts = lib.splitString needle haystack;
+    in
+    if builtins.length parts < 2 then null else builtins.stringLength (builtins.head parts);
+in
+lib.genAttrs (builtins.attrNames outputs.nixosConfigurations) (
+  name:
+  let
+    cfg = outputs.nixosConfigurations.${name}.config;
+    rules = cfg.networking.firewall.extraInputRules;
+    dropAt = firstIndex "saddr 10.0.0.0/8 tcp dport" rules;
+    acceptAt = firstIndex "saddr 10.0.0.0/8 accept" rules;
+  in
+  if !(cfg.services.k3s.enable or false) then
+    true
+  else
+    # The port deny must exist and must precede the broad pod accept, because
+    # nftables takes the first matching rule.
+    dropAt != null
+    && acceptAt != null
+    && dropAt < acceptAt
+    && lib.hasInfix "2049" rules
+    && lib.hasInfix "5432" rules
+    && lib.hasInfix "5902" rules
+)