nixos/test-driver: add backdoor based on systemd-ssh-proxy & AF_VSOCK

With this it's possible to trivially SSH into running machines from the
test-driver. This is especially useful when running VM tests
interactively on a remote system.

This is based on `systemd-ssh-proxy(1)`, so there's no need to configure
any additional networking on the host-side.

Suggested-by: Ryan Lahfa <masterancpp@gmail.com>

nixos/doc/manual/development/running-nixos-tests-interactively.section.md

@@ -63,6 +63,35 @@ using:
 Once the connection is established, you can enter commands in the socat terminal
 where socat is running.
 
+## SSH Access for test machines {#sec-nixos-test-ssh-access}
+
+An SSH-based backdoor to log into machines can be enabled with
+
+```nix
+{
+  name = "…";
+  nodes.machines = { /* … */ };
+  sshBackdoor.enable = true;
+}
+```
+
+This creates a [vsock socket](https://man7.org/linux/man-pages/man7/vsock.7.html)
+for each VM to log in with SSH. This configures root login with an empty password.
+
+When the VMs get started interactively with the test-driver, it's possible to
+connect to `machine` with
+
+```
+$ ssh vsock/3 -o User=root
+```
+
+The socket numbers correspond to the node number of the test VM, but start
+at three instead of one because that's the lowest possible
+vsock number.
+
+On non-NixOS systems you'll probably need to enable
+the SSH config from {manpage}`systemd-ssh-proxy(1)` yourself.
+
 ## Port forwarding to NixOS test VMs {#sec-nixos-test-port-forwarding}
 
 If your test has only a single VM, you may use e.g.

nixos/doc/manual/redirects.json

@@ -1817,6 +1817,9 @@
   "sec-test-options-reference": [
     "index.html#sec-test-options-reference"
   ],
+  "test-opt-sshBackdoor.enable": [
+    "index.html#test-opt-sshBackdoor.enable"
+  ],
   "test-opt-defaults": [
     "index.html#test-opt-defaults"
   ],
@@ -1904,6 +1907,9 @@
   "sec-nixos-test-shell-access": [
     "index.html#sec-nixos-test-shell-access"
   ],
+  "sec-nixos-test-ssh-access": [
+    "index.html#sec-nixos-test-ssh-access"
+  ],
   "sec-nixos-test-port-forwarding": [
     "index.html#sec-nixos-test-port-forwarding"
   ],

nixos/lib/testing/nodes.nix

@@ -13,6 +13,7 @@ let
     mapAttrs
     mkDefault
     mkIf
+    mkMerge
     mkOption
     mkForce
     optional
@@ -77,6 +78,14 @@ in
 {
 
   options = {
+    sshBackdoor = {
+      enable = mkOption {
+        default = false;
+        type = types.bool;
+        description = "Whether to turn on the vsock-based SSH backdoor for all VMs.";
+      };
+    };
+
     node.type = mkOption {
       type = types.raw;
       default = baseOS.type;
@@ -172,10 +181,15 @@ in
 
     passthru.nodes = config.nodesCompat;
 
-    defaults = mkIf config.node.pkgsReadOnly {
-      nixpkgs.pkgs = config.node.pkgs;
-      imports = [ ../../modules/misc/nixpkgs/read-only.nix ];
-    };
+    defaults = mkMerge [
+      (mkIf config.node.pkgsReadOnly {
+        nixpkgs.pkgs = config.node.pkgs;
+        imports = [ ../../modules/misc/nixpkgs/read-only.nix ];
+      })
+      (mkIf config.sshBackdoor.enable {
+        testing.sshBackdoor.enable = true;
+      })
+    ];
 
   };
 }

nixos/modules/testing/test-instrumentation.nix

@@ -87,6 +87,10 @@ in
       machine.switch_root() to leave stage 1 and proceed to stage 2
     '';
 
+    sshBackdoor = {
+      enable = mkEnableOption "vsock-based ssh backdoor for the VM";
+    };
+
   };
 
   config = {
@@ -100,6 +104,18 @@ in
       }
     ];
 
+    services.openssh = mkIf config.testing.sshBackdoor.enable {
+      enable = true;
+      settings = {
+        PermitRootLogin = "yes";
+        PermitEmptyPasswords = "yes";
+      };
+    };
+
+    security.pam.services.sshd = mkIf config.testing.sshBackdoor.enable {
+      allowNullPassword = true;
+    };
+
     systemd.services.backdoor = lib.mkMerge [
       backdoorService
       {
@@ -175,6 +191,10 @@ in
         #       we avoid defining attributes if not possible.
         # TODO: refactor such that test-instrumentation can import qemu-vm
         package = lib.mkDefault pkgs.qemu_test;
+
+        options = mkIf config.testing.sshBackdoor.enable [
+          "-device vhost-vsock-pci,guest-cid=${toString (config.virtualisation.test.nodeNumber + 2)}"
+        ];
       };
     };