nixos/test-driver: add backdoor based on systemd-ssh-proxy & AF_VSOCK (#392030)

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

···
 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

···
   "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"
   ],
···
   ],
   "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/test-driver/src/test_driver/__init__.py

···
         help="the test script to run",
         type=Path,
     )
+    arg_parser.add_argument(
+        "--dump-vsocks",
+        help="indicates that the interactive SSH backdoor is active and dumps information about it on start",
+        action="store_true",
+    )
 
     args = arg_parser.parse_args()
 
···
         if args.interactive:
             history_dir = os.getcwd()
             history_path = os.path.join(history_dir, ".nixos-test-history")
+            if args.dump_vsocks:
+                driver.dump_machine_ssh()
             ptpython.ipython.embed(
                 user_ns=driver.test_symbols(),
                 history_filename=history_path,

nixos/lib/test-driver/src/test_driver/driver.py

···
 from typing import Any
 from unittest import TestCase
 
+from colorama import Style
+
 from test_driver.errors import MachineError, RequestedAssertionFailed
 from test_driver.logger import AbstractLogger
 from test_driver.machine import Machine, NixStartScript, retry
···
             + ", ".join(list(general_symbols.keys()))
         )
         return {**general_symbols, **machine_symbols, **vlan_symbols}
+
+    def dump_machine_ssh(self) -> None:
+        print("SSH backdoor enabled, the machines can be accessed like this:")
+        print(
+            f"{Style.BRIGHT}Note:{Style.RESET_ALL} this requires {Style.BRIGHT}systemd-ssh-proxy(1){Style.RESET_ALL} to be enabled (default on NixOS 25.05 and newer)."
+        )
+        names = [machine.name for machine in self.machines]
+        longest_name = len(max(names, key=len))
+        for num, name in enumerate(names, start=3):
+            spaces = " " * (longest_name - len(name) + 2)
+            print(
+                f"    {name}:{spaces}{Style.BRIGHT}ssh -o User=root vsock/{num}{Style.RESET_ALL}"
+            )
 
     def test_script(self) -> None:
         """Run the test script"""

nixos/lib/testing-python.nix

···
           ),
         extraPythonPackages ? (_: [ ]),
         interactive ? { },
+        sshBackdoor ? { },
       }@t:
       let
         testConfig =

nixos/lib/testing/nodes.nix

···
     mapAttrs
     mkDefault
     mkIf
+    mkMerge
     mkOption
     mkForce
     optional
···
 {
 
   options = {
+    sshBackdoor = {
+      enable = mkOption {
+        default = false;
+        type = types.bool;
+        description = "Whether to turn on the VSOCK-based access to all VMs. This provides an unauthenticated access intended for debugging.";
+      };
+    };
+
     node.type = mkOption {
       type = types.raw;
       default = baseOS.type;
···
 
     passthru.nodes = config.nodesCompat;
 
-    defaults = mkIf config.node.pkgsReadOnly {
-      nixpkgs.pkgs = config.node.pkgs;
-      imports = [ ../../modules/misc/nixpkgs/read-only.nix ];
-    };
+    extraDriverArgs = mkIf config.sshBackdoor.enable [
+      "--dump-vsocks"
+    ];
+
+    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

···
       machine.switch_root() to leave stage 1 and proceed to stage 2
     '';
 
+    sshBackdoor = {
+      enable = mkEnableOption "vsock-based ssh backdoor for the VM";
+    };
+
   };
 
   config = {
···
         '';
       }
     ];
+
+    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
···
         #       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)}"
+        ];
       };
     };