Keep places and accesses at the terminal, and refuse a line break in any setting

Through the settings verb, or a settings line run by the generic command
verb, any caller of the mesh's console could place a module's directory at
/etc with an owner of its own and have the node-engine, as root, hand it
over at the next push, or mount any of the machine's paths into a container
(hq issue 339). A change to either key is now refused in every process a
verb runs, the generic verb refuses settings writes outright, and neither key
may name the machine's own trees from anywhere, the terminal included. A
line break, carriage return or NUL in any setting, which a file it is
written into reads as a line of the caller's own, is refused where a layer
is kept and where it is composed; PEM blocks alone may hold lines.

cmd/mesh-controller/modules.go

@@ -445,6 +445,9 @@ func settingsCommand(ctx context.Context, args []string) error {
 		if err != nil {
 			return err
 		}
+		if err := refuseTerminalSettingsThroughAVerb(before, values, positionals[0], where); err != nil {
+			return err
+		}
 		added, changed, removed := settingsChange(before, values)
 		if len(removed) > 0 && !*replace {
 			return fmt.Errorf("%s on %s: this layer would no longer set %s. A layer is replaced whole; "+
@@ -596,6 +599,13 @@ func settingsCommand(ctx context.Context, args []string) error {
 		if len(positionals) != 1 {
 			return errors.New("settings clear <module> [--node <node>]")
 		}
+		before, _, err := inv.Layer(ctx, *node, positionals[0])
+		if err != nil {
+			return err
+		}
+		if err := refuseTerminalSettingsThroughAVerb(before, nil, positionals[0], where); err != nil {
+			return err
+		}
 		if err := inv.ClearSettings(ctx, *node, positionals[0]); err != nil {
 			return err
 		}
@@ -1051,3 +1061,40 @@ func declaresTools(m catalogue.Manifest) bool {
 	}
 	return false
 }
+
+// terminalSettings are the keys no verb may change (novox/hq issue 339). `places` says where the node-engine
+// creates and, as root, owns a module's directories, with an owner the setting names; `accesses` says which of
+// the machine's paths are mounted into a module's container. Set through a verb, either lets any caller of the
+// mesh's console — an agent among them — have root hand it a directory, or mount one of the machine's into a
+// container it reaches. They are the operator's, typed at the controller's terminal.
+var terminalSettings = []string{catalogue.PlacesSetting, catalogue.AccessesSetting}
+
+// throughAVerb says whether this process runs a seat verb's command line: the serving controller names the
+// verb in the environment of every command it runs for one (runVerb), and a person at the terminal runs none.
+// Every verb route reaches a command through runVerb — the named verbs and the generic `command` alike — so
+// this is the one place that knows, whatever line the verb composed.
+func throughAVerb() (string, bool) {
+	verb := os.Getenv(verbVar)
+	return verb, verb != ""
+}
+
+// refuseTerminalSettingsThroughAVerb refuses a layer change through a verb that would add, change or remove
+// places or accesses; a change that leaves both as they were is not refused.
+func refuseTerminalSettingsThroughAVerb(before, after map[string]any, module, where string) error {
+	verb, through := throughAVerb()
+	if !through {
+		return nil
+	}
+	for _, key := range terminalSettings {
+		was, _ := json.Marshal(before[key])
+		now, _ := json.Marshal(after[key])
+		if string(was) == string(now) {
+			continue
+		}
+		return fmt.Errorf("%s of %s on %s is set at the controller's terminal only, never through a verb (this "+
+			"line came through %q): it says where root creates and owns a module's directories, or which of "+
+			"the machine's paths are mounted into its container, and whoever may call a verb includes agents "+
+			"(novox/hq issue 339). Nothing was changed", key, module, where, verb)
+	}
+	return nil
+}