NoPerformanceNow.user.js

// ==UserScript==
// @name         NoPerformanceNow
// @namespace    https://yuki.network/
// @version      0.2
// @description  Disables performance.now() to make JavaScript exploits harder.
// @author       ReimuNotMoe, ClassicOldSong
// @match        *://*/*
// @grant        unsafeWindow
// ==/UserScript==

// This program is free software: you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.

// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.

// You should have received a copy of the GNU General Public License
// along with this program.  If not, see <http://www.gnu.org/licenses/>.

const oldConsoleError = console.error.bind(console);
const oldNow = performance.now;
const oldToLocalString = Function.prototype.toLocalString;
const oldToString = Function.prototype.toString;
const oldToSource = Function.prototype.toSource;

const originalFns = [];
const overwrittenFns = [];

let fakeNow = 0;

const now = () => {
    oldConsoleError("performance.now() is being used");
    return fakeNow += Math.random() * 1000;
};

function toLocalString(...args) {
    const idx = overwrittenFns.indexOf(this);
    if (idx >= 0) return oldToLocalString.call(originalFns[idx]);
    return oldToLocalString.call(this, ...args);
}

function toString(...args) {
    const idx = overwrittenFns.indexOf(this);
    if (overwrittenFns.indexOf(this) >= 0) return oldToString.call(originalFns[idx]);
    return oldToString.call(this, ...args);
};

function toSource(...args) {
    const idx = overwrittenFns.indexOf(this);
    if (overwrittenFns.indexOf(this) >= 0) return oldToSource.call(originalFns[idx]);
    return oldToSource.call(this, ...args);
};

originalFns.push(performance.now, oldToLocalString, oldToString, oldToSource);
overwrittenFns.push(now, toLocalString, toString, toSource);

(function() {
    'use strict';

    unsafeWindow.performance.__proto__.now = now;
    if (oldToLocalString) Function.prototype.toLocalString = toLocalString;
    if (oldToString) Function.prototype.toString = toString;
    if (oldToSource) Function.prototype.toSource = toSource;
})();