endo: iOS Safari fails to lockdown (with potential fix)

1. Call lockdown() on the latest version of iOS Safari
2. Lockdown fails with the error Cannot read "configurable" of undefined in the console

Digging in a bit, I found that the error is caused inside isImmuatableDataProperty():

function isImmutableDataProperty(obj, name) {
  const desc = getOwnPropertyDescriptor(obj, name)
  return (
    //
    // The getters will not have .writable, don't let the falsyness of
    // 'undefined' trick us: test with === false, not ! . However descriptors
    // inherit from the (potentially poisoned) global object, so we might see
    // extra properties which weren't really there. Accessor properties have
    // 'get/set/enumerable/configurable', while data properties have
    // 'value/writable/enumerable/configurable'.
    desc.configurable === false &&
    desc.writable === false &&
    //
    // Checks for data properties because they're the only ones we can
    // optimize (accessors are most likely non-constant). Descriptors can't
    // can't have accessors and value properties at the same time, therefore
    // this check is sufficient. Using explicit own property deal with the
    // case where Object.prototype has been poisoned.
    objectHasOwnProperty(desc, 'value')
  )
}

It is called with isImmutableDataProperty(window, 'showModalDialog'), which in iOS safari is actually undefined. So when it tries to get the property descriptor, desc is also undefined.

I added a hack to return true if desc is undefined and everything seems to work as expected. I’m not sure if that is the actual fix though.