[MuffinFlavored]

I know one of WebAssembly's biggest features by design is security / "sandbox".

But I've always gotten confused with... it is secure because by default it can't do much.

I don't quite understand how to view WebAssembly. You write in one language, it compiles things like basic math (nothing with network or filesystem) to another and it runs in an interpreter.

I feel like I have a severe lack/misunderstanding. There's a ton of hype for years, lots of investment... but it isn't like any case where you want to add Lua to an app you can add WebAssembly/vice versa?

[jeroenhd]

WebAssembly can communicate through buffers. WebAssembly can also import foreign functions (Javascript functions in the browser).

You can get output by reading the buffer at the end of execution/when receiving callbacks. So, for instance, you pass a few frames worth of buffers to WASM, WASM renders pixels into the buffers, calls a callback, and the Javascript reads data from the buffer (sending it to a <canvas> or similar).

The benefit of WASM is that it can't be very malicious by itself. It requires the runtime to provide it with exported functions and callbacks to do any file I/O, network I/O, or spawning new tasks. Lua and similar tools can go deep into the runtime they exist in, altering system state and messing with system memory if they want to, while WASM can only interact with the specific API surface you provide it.

That makes WASM less powerful, but more predictable, and in my opinion better for building integrations with as there is no risk of internal APIs being accessed (that you will be blamed for if they break in an update).

[brabel]

> Lua and similar tools can go deep into the runtime they exist in, altering system state and messing with system memory if they want to

That's not correct, when you embed Lua you can choose which APIs are available, to make the full stdlib available you must explicitly call `luaL_openlibs` [1].

[1] https://www.lua.org/manual/5.3/manual.html#luaL_openlibs

[westurner]

WASI Preview 1 and WASI Preview 2 can do file and network I/O IIUC.

Re: tty support in container2wasm and fixed 80x25 due to lack of SIGWINCH support in WASI Preview 1: https://github.com/ktock/container2wasm/issues/146

The File System Access API requires granting each app access to each folder.

jupyterlab-filesystem-access only works with Chromium based browsers, because FF doesn't support the File System Access API: https://github.com/jupyterlab-contrib/jupyterlab-filesystem-...

The File System Access API is useful for opening a local .ipynb and .csv with JupyterLite, which builds CPython for WASM as Pyodide.

There is a "Direct Sockets API in Chrome 131" but not in FF; so WebRTC and WebSocket relaying is unnecessary for WASM apps like WebVM: https://news.ycombinator.com/item?id=42029188

[westurner]

WASI Preview 2: https://github.com/WebAssembly/WASI/blob/main/wasip2/README.... :

> wasi-io, wasi-clocks, wasi-random, wasi-filesystem, wasi-sockets, wasi-cli, wasi-http

[panic]

I don’t believe it is currently possible for a WebAssembly instance to access any buffer other than its own memory. You have to copy data in and out.

[deathanatos]

The embedder could hand the module functions for manipulating external buffers via externrefs. (I'm not sure if that's a good idea, or not, just that it could.)

But if the module wants to compute on the values in the buffer, at some level it would have to copy the data in/out.

[davexunit]

Use the GC instructions and you can freely share heap references amongst other modules and the host.

[panic]

How do you access the contents of a heap reference from JavaScript in order to “send it to a <canvas> or similar”?

[davexunit]

Assuming you're talking about reading binary data like (array i8), the GC MVP doesn't have a great answer right now. Have to call back into wasm to read the bytes. Something for the group to address in future proposals. Sharing between wasm modules is better right now.

[pdubroy]

You should check out the book :-)

We have a chapter called "What Makes WebAssembly Safe?" which covers the details. You can get a sneak peek here: https://bsky.app/profile/wasmgroundup.com/post/3lh2e4eiwnm2p

[pizlonator]

> But I've always gotten confused with... it is secure because by default it can't do much.

Yes. That’s a super accurate description. You’re not confused.

> I don't quite understand how to view WebAssembly. You write in one language, it compiles things like basic math (nothing with network or filesystem) to another and it runs in an interpreter.

Almost. Wasm is cheap to JIT compile and the resulting code is usually super efficient. Sometimes parity with native execution.

> I feel like I have a severe lack/misunderstanding. There's a ton of hype for years, lots of investment... but it isn't like any case where you want to add Lua to an app you can add WebAssembly/vice versa?

It’s definitely a case where the investment:utility ratio is high. ;-)

Here’s the trade off between embedding Lua and embedding Wasm:

- Both have the problem that they are only as secure as the API you expose to the guest program. If you expose `rm -rf /` to either Lua or Wasm, you’ll have a bad time. And it’s surprisingly difficult to convince yourself that you didn’t accidentally do that. Security is hard.

- Wasm is faster than Lua.

- Lua is a language for humans, no need for another language and compiler. That makes Lua a more natural choice for embedded scripting.

- Lua is object oriented, garbage collected, and has a very principled story for how that gets exposed to the host in a safe way. Wasm source languages are usually not GC’d. That means that if you want to expose object oriented API to the guest program, then it’ll feel more natural to do that with Lua.

- The wasm security model is dead simple and doesn’t (necessarily) rely on anything like GC, making it easier to convince yourself that the wasm implementation is free of security vulnerabilities. If you want a sandboxed execution environment then Wasm is better for that reason.

[Karellen]

> You write in one language

Not quite. Web assembly isn't a source language, it's a compiler target. So you should be able to write in C, Rust, Fortran, or Lua and compile any of those to WebAssembly.

Except that WebAssembly is a cross-platform assembly language/machine code which is very similar to the native machine code of many/most contemporary CPUs. This means a WebAssembly interpreter can be very straightforward, and could often translate one WebAssembly instruction to one native CPU instruction. Or rather, it can compile a stream of WebAssembly instructions almost one-to-one to native CPU instructions, which it can then execute directly.

[whizzter]

A JIT should be able to translate most arithmetic and binary instructions to single-opcodes, however anything involving memory and functions calls needs safety checks that becomes multi-instruction. branches could mostly be direct _unless_ the runtime has any kind of metering (it should) to stop eternal loops (if it also wants to be crash-safe even if it's exploit safe).

[kouteiheika]

> anything involving memory [..] needs safety checks that becomes multi-instruction

Not necessarily; on AMD64 you can do memory accesses in a single instruction relatively easily by using the CPU's paging machinery for safety checks plus some clever use of address space.

> branches could mostly be direct _unless_ the runtime has any kind of metering (it should) to stop eternal loops

Even with metering the branches would be direct, you'd just insert the metering code at the start of each basic block (so that's two extra instructions at the start of each basic block). Or did you mean something else?

[whizzter]

Can't remember exactly what one but I remember reading an article about some VM that added interruption checks not at block boundaries, but rather only at _backwards_ branches and call-sites, so "safe" forward jumps (if/else/break) wouldn't cost anything extra but anything that could go on "forever" had the checks.

Reserving 4gb address space oughta work on any 64bit machine with a decent OS/paging system though? I was looking into it but couldn't use it in my case however since it needs to cooperate with another VM that already hooks the PF handler (although maybe I should take another stab if there is a way to put in a hierarhcy).

[charleslmunger]

Metering also doesn't require a branch if you implement it with page faults. See "Implicit suspend checks" in https://android-developers.googleblog.com/2023/11/the-secret...

[kouteiheika]

Yep. That's a nice trick; unfortunately it's non-deterministic.

[csjh]

How's it nondeterministic?

[dist1ll]

How do you avoid branches in 64-bit WASM?

[kouteiheika]

You can run the guest in another thread/process and give it its own dedicated address space, or use something like memory protection keys.

[beardyw]

Yes, interpretation on the fly was never its intention. The intention was to provide interpreted languages with a way to implement fast compiled functions.

[Gupta2]

Speaking of WebAssembly security, is it vulnerable to Spectre/CPU style attacks like those in JavaScript? (WASM without imported JS functions)

[jeffparsons]

Yes, if you give the Wasm instance access to timers.

[saagarjha]

Yes.

[coliveira]

I think the biggest advantage of wasm in terms of security is that it doesn't accept machine language written in the target machine, only in this artificial machine language. This means that it cannot encode arbitrary code that could be executed by the host machine. Everything it runs has necessarily to go through the wasm interpreter.

[wyldfire]

> This means that it cannot encode arbitrary code that could be executed by the host machine.

But the host machine still can, so it's not as big of advantage in that regard. If you could somehow deliver a payload of native code and jump to it, it'd work just fine. But the security you get is the fact that it's really hard to do that because there's no wasm instructions to jump to arbitrary memory locations (even if all the host ISAs do have those). Having a VM alone doesn't provide security against attacks.

It's often the case that VMs are used with memory-safe languages and those languages' runtime bounds checks and other features are what gives them safety moreso than their VM. In fact, most bytecode languages provide a JIT (including some wasm deployments) so you're actually running native code regardless.

[hb-robo]

That's quite interesting. This is way outside of my wheelhouse - has this kind of approach been tried in other security contexts before? What would you even call that, virtualization?

[dmitrygr]

The word is "bytecode" and the idea is as old as computing.

[tubs]

Java.

[jedisct1]

It's like JavaScript, Python or PHP. There are no pointers, only arrays, that cause the app to crash if it attempts to read out of the bounds.