War story: the hardest bug I ever debugged

> which can be abused to rewrite the array’s length and enable further shenanigans.

I followed all of this up until here. JavaScript lets you modify the length of an array by assigning to indexes that are negative? I'm familiar with the paradigm of negative indexing being used to access things from the end of the array (like -1 being the last element), but I don't understand what operation someone could do that would somehow modify the length of the array rather than modifying a specific element in-place. Does JIT-compiled JavaScript not follow the usual JavaScript semantics that would normally happen when using a negative index, or are you describing something that would be used in combination with some other compiler bug (which honestly sounds a lot more severe even in the absence of an usual Math.abs implementation).

This is after the jit.

I.e. don't think fancy language shenanigans that do negative indexing. But negative offset from the beginning of the array memory access.

When there's some inlining, there will be no function call into some index operator function

>I followed all of this up until here. JavaScript lets you modify the length of an array by assigning to indexes that are negative?

This is my no doubt dumb understanding of what you can do, based on some funky stuff I did one time to mess with people's heads

do the following const arr = []; arr[-1] = "hi"; console.log(arr) this gives you "-1": "hi"

length: 0

which I figured is because really an array is just a special type of object. (my interpretation, probably wrong)

now we can see that the JavaScript Array length is 0, but since the value is findable in there I would expect there is some length representation in the lower level language that JavaScript is implemented in, in the browser, and I would then think that there could even be exploits available by somehow taking advantage of the difference between this lower level representation of length and the JS array length. (again all this is silly stuff I thought and have never investigated, and is probably laughably wrong in some ways)

I remember seeing some additions to array a few years back that made it so you could protect against the possibility of negative indexes storing data in arrays - but that memory may be faulty as I have not had any reason to worry about it.

Normally, there would be a bounds check to ensure that the index was actually non-negative; negative indices get treated as property accesses instead of array accesses (unlike e.g. Python where they would wrap around).

However, if the JIT compiler has "proven" that the index is never non-negative (because it came from Math.abs), it may omit such checks. In that case, the resulting access to e.g. arr[-1] may directly access the memory that sits one position before the array elements - which could, for example, be part of the array metadata, such as the length of the array.

You can read the comments on the sample CVE's proof-of-concept to see what the JS engine "thinks" is happening, vs. what actually happens when the code is executed: https://github.com/shxdow/exploits/blob/master/CVE-2020-9802.... This exploit is a bit more complicated than my description, but uses a similar core idea.

Javascript is the new Macromedia/Adobe Flash.

You can do more and more in it and it's so fun, until it suddenly isn't anymore and dies.

For example if arrays were implemented like this (they're not)

    struct js_array {
        uint64_t length;
        js_value *values[];
    }

Because after bound checks have been taken care of, loading an element of a JS array probably compiles to a simple assembly-level load like mov. If you bypass the bounds checks, that mov can read or write any mapped address.

Yeah, I understand all of that. I think my surprise was that you can access arbitrary parts of this struct from within JavaScript at all; I guess I really just haven't delved deeply enough into what JIT compiling actually is doing at runtime, because I wouldn't have expected that to be possible.

I understand the idea of the lack of a bounds check allowing access to early memory with a negative index, but I'm mostly struggling with wrapping my head around why the underlying memory layout is accessible in JavaScript in the first place. I hadn't considered the fact that the same syntax could be used for accessing arbitrary properties rather than just array indexes; that might be the nuance I was missing.

You raise a good point that JavaScript arrays are "just" objects that let you assign to arbitrary properties through the same syntax as array indexing. I could totally imagine some sort of optimization where a compiler utilizes this to be able to map arrays directly to their underlying memory layout (presumably with a length prefix), and that would end up potentially providing access to it in the case of a mistaken assumption about omitting a bounds check.

yeah you know what you said made me think about these funny experiments that I haven't done in a long time and I remember now yeah, you can do

const arr = []; arr[false] = "hi";

which console.log(arr); - in FF at least - gives

Array []

false: "hi"

length: 0

which means

console.log(arr[Boolean(arr.length)]); returns

hi

which is funny, I just feel there must be an exploit somewhere among this area of things, but maybe not because it would be well covered.

on edit: for example since the index could be achieved - for some reason - from numeric operation that output NaN, you would then have NaN: "hi", or since the arr[-1] gives you "-1": "hi" but arr[0 -1] returns that "hi" there are obviously type conversions going on in the indexing...which just always struck me as a place you don't expect the type conversions to be going on the way you do with a == b;

Maybe I am just easily freaked out by things as I get older.