settings: validate hostnames from D-Bus (bgo #711179)

Do some minimal verification of hostnames that come in via D-Bus, for
length and content.  Otherwise we'd get as far as asking glibc to set
the system hostname, which would reject us.

src/settings/nm-settings-error.h

@@ -40,6 +40,7 @@ typedef enum {
 	NM_SETTINGS_ERROR_ADD_FAILED,                  /*< nick=AddFailed >*/
 	NM_SETTINGS_ERROR_SAVE_HOSTNAME_NOT_SUPPORTED, /*< nick=SaveHostnameNotSupported >*/
 	NM_SETTINGS_ERROR_SAVE_HOSTNAME_FAILED,        /*< nick=SaveHostnameFailed >*/
+	NM_SETTINGS_ERROR_HOSTNAME_INVALID,            /*< nick=HostnameInvalid >*/
 	NM_SETTINGS_ERROR_UUID_EXISTS,                 /*< nick=UuidExists >*/
 } NMSettingsError;
 

src/settings/nm-settings.c

@@ -1356,6 +1356,33 @@ pk_hostname_cb (NMAuthChain *chain,
 	nm_auth_chain_unref (chain);
 }
 
+static gboolean
+validate_hostname (const char *hostname)
+{
+	const char *p;
+	gboolean dot = TRUE;
+
+	if (!hostname || !hostname[0])
+		return FALSE;
+
+	for (p = hostname; *p; p++) {
+		if (*p == '.') {
+			if (dot)
+				return FALSE;
+			dot = TRUE;
+		} else {
+			if (!g_ascii_isalnum (*p) && (*p != '-') && (*p != '_'))
+				return FALSE;
+			dot = FALSE;
+		}
+	}
+
+	if (dot)
+		return FALSE;
+
+	return (p - hostname <= HOST_NAME_MAX);
+}
+
 static void
 impl_settings_save_hostname (NMSettings *self,
                              const char *hostname,
@@ -1365,6 +1392,14 @@ impl_settings_save_hostname (NMSettings *self,
 	NMAuthChain *chain;
 	GError *error = NULL;
 
+	/* Minimal validation of the hostname */
+	if (!validate_hostname (hostname)) {
+		error = g_error_new_literal (NM_SETTINGS_ERROR,
+		                             NM_SETTINGS_ERROR_HOSTNAME_INVALID,
+		                             "The hostname was too long or contained invalid characters.");
+		goto done;
+	}
+
 	/* Do any of the plugins support setting the hostname? */
 	if (!get_plugin (self, NM_SYSTEM_CONFIG_INTERFACE_CAP_MODIFY_HOSTNAME)) {
 		error = g_error_new_literal (NM_SETTINGS_ERROR,