WIP: mutable user eval test

nixos/module-tests/mutable-users.nix

@@ -0,0 +1,75 @@
+let
+  ensurePass = name: config:
+    builtins.seq
+      (builtins.unsafeDiscardOutputDependency
+        (import ../lib/eval-config.nix {
+          modules = [
+            config
+            ({
+              fileSystems."/".device = "/dev/bogus";
+              boot.loader.grub.device = "/dev/bogus";
+            })
+          ];
+        }).config.system.build.toplevel.drvPath
+      ) "ok" ;
+
+
+    ensureFail = name: config:
+      if (builtins.tryEval (ensurePass name config)).success == false
+      then "ok"
+      else throw "unexpected success in ${name}";
+in {
+  user-with-password = ensurePass "user-with-password" {
+    # services.openssh.enable = true;
+    users.mutableUsers = false;
+    users.users.foo = {
+      password = "foob";
+      extraGroups = [ "wheel" ];
+    };
+    users.users.root.hashedPassword = null;
+  };
+
+  root-only-no-password = ensureFail "root-only-no-password" {
+    users.mutableUsers = false;
+    users.users.root.hashedPassword = null;
+  };
+
+  root-only-with-bang-password = ensureFail "root-only-with-bang-password" {
+    users.mutableUsers = false;
+    users.users.root.hashedPassword = "!";
+  };
+
+  root-only-with-real-password = ensurePass "root-only-with-real-password" {
+    users.mutableUsers = false;
+    users.users.root.hashedPassword = "w00t";
+  };
+
+  root-only-with-real-password-ssh-no-login = ensureFail "root-only-with-real-password-ssh-no-login" {
+    users.mutableUsers = false;
+    users.users.root.hashedPassword = "w00t";
+    services.openssh.enable = true;
+    services.openssh.permitRootLogin = "no";
+  };
+
+  root-only-with-real-password-ssh-without-pass-login = ensureFail "root-only-with-real-password-ssh-without-pass-login" {
+    users.mutableUsers = false;
+    users.users.root.hashedPassword = "w00t";
+    services.openssh.enable = true;
+    services.openssh.permitRootLogin = "without-password";
+  };
+
+  root-only-with-real-password-ssh-prohibit-password-login = ensureFail "root-only-with-real-password-ssh-prohibit-password-login" {
+    users.mutableUsers = false;
+    users.users.root.hashedPassword = "w00t";
+    services.openssh.enable = true;
+    services.openssh.permitRootLogin = "prohibit-password";
+  };
+
+  root-only-with-real-password-ssh-no-login-ssh-disabled = ensurePass "root-only-with-real-password-ssh-no-login-ssh-disabled" {
+    users.mutableUsers = false;
+    users.users.root.hashedPassword = "w00t";
+    services.openssh.enable = false;
+    services.openssh.permitRootLogin = "no";
+  };
+
+}

nixos/modules/config/users-groups.nix

@@ -557,18 +557,77 @@ in {
         # password or an SSH authorized key. Privileged accounts are
         # root and users in the wheel group.
         assertion = !cfg.mutableUsers ->
-          any id (mapAttrsToList (name: cfg:
-            (name == "root"
-             || cfg.group == "wheel"
-             || elem "wheel" cfg.extraGroups)
-            &&
-            ((cfg.hashedPassword != null && cfg.hashedPassword != "!")
-             || cfg.password != null
-             || cfg.passwordFile != null
-             || cfg.openssh.authorizedKeys.keys != []
-             || cfg.openssh.authorizedKeys.keyFiles != [])
-          ) cfg.users);
+          (let
+            userHasKeys = cfg:
+              (
+                cfg.openssh.authorizedKeys.keys != []
+                || cfg.openssh.authorizedKeys.keyFiles != []
+              );
+
+            userHasPassword = cfg:
+              (
+                (cfg.hashedPassword != null && cfg.hashedPassword != "!")
+                || cfg.password != null
+                || cfg.passwordFile != null
+              );
+
+            userHasSSHCreds = name: cfg:
+              let
+                ssh = config.services.openssh;
+                passwordWorks = if ssh.passwordAuthentication
+                  then userHasPassword cfg
+                  else false;
+
+                keyWorks = userHasKeys cfg;
+              in if name == "root" then
+                (
+                  # Is root allowed explicitly to use a password?
+                  if ssh.permitRootLogin == "yes"
+                    then passwordWorks || keyWorks
+                  else
+                  # Is root allowed explicitly denied from using a password?
+                  if (ssh.permitRootLogin == "without-password"
+                      || ssh.permitRootLogin == "prohibit-password")
+                    then keyWorks
+                  else
+                  # Forced commands don't count as being allowed in
+                  # and no means no
+                  if (ssh.permitRootLogin == "forced-command"
+                      || ssh.permitRootLogin == "no")
+                      then false
+                  else
+                    builtins.trace ("Cannot handle openssh.permitRootLogin"
+                      + " = ${ssh.permitRootLogin} in determining if it"
+                      + " can be used for SSH login. Assuming no, for"
+                      + " safety.") false
+                )
+              else passwordWorks || keyWorks;
+
+            userIsWheel = cfg:
+              (
+                cfg.group == "wheel"
+                || elem "wheel" cfg.extraGroups
+              );
+
+            userCanBecomeRoot = name: cfg:
+              if name == "root"
+                then true
+              else if config.security.sudo.enable == false
+                then false
+              else if config.security.sudo.wheelNeedsPassword
+                then (userIsWheel cfg && userHasPassword cfg)
+              else userIsWheel cfg;
+
+          in any id (mapAttrsToList (name: cfg:
+            userCanBecomeRoot name cfg
+            && (
+              if config.services.openssh.enable
+                then userHasSSHCreds name cfg
+                else userHasPassword cfg
+            )
+          ) cfg.users));
         message = ''
+          No account can log in at the console or over SSH. Please set
           Neither the root account nor any wheel user has a password or SSH authorized key.
           You must set one to prevent being locked out of your system.'';
       }

nixos/modules/services/networking/ssh/sshd.nix

@@ -21,7 +21,7 @@ let
           daemon reads in addition to the the user's authorized_keys file.
           You can combine the <literal>keys</literal> and
           <literal>keyFiles</literal> options.
-          Warning: If you are using <literal>NixOps</literal> then don't use this 
+          Warning: If you are using <literal>NixOps</literal> then don't use this
           option since it will replace the key required for deployment via ssh.
         '';
       };
@@ -114,6 +114,8 @@ in
 
       permitRootLogin = mkOption {
         default = "prohibit-password";
+        # When changing the allowed types, you must also update
+        # userHasSSHCreds in nixos/modules/config/users-groups.nix!
         type = types.enum ["yes" "without-password" "prohibit-password" "forced-commands-only" "no"];
         description = ''
           Whether the root user can login using ssh.