multi-memory VS wajic

Support for multi-memory to deal with multiple memories in Wasm.

> You can do attacks that most people haven't been able to do for 20+ years.

This is a bad and roundabout way to say that vulnerabilities in WebAssembly modules may cause a corruption in their linear memory. Which is absolutely true, but those attacks still matter today (not everyone turns ASLR on) and similar defences also apply. In the future multiple memories [1] should make it much easier to guard against remaining issues. WebAssembly is a lucrative target only because it is so widespread, not because it has horrible security (you don't know what the actually horrible security looks like).

[1] https://github.com/WebAssembly/multi-memory/blob/main/propos...

Thanks! These claims are really interesting.

- WASM has no ASLR.

So I guess if a buffer overrun lets you modify a function pointer, you could replace that function pointer with another pointer to execute different code. As you say, this is hard in native linux programs because ASLR and NX. You need a pointer to some code thats loaded in memory and you need to know where it is. In wasm, the "pointer" isn't a pointer at all. indirect_call takes an index into the jump table. Yes, this makes it easier to find other valid function pointers. But wasm also has some advantages here. Unlike in native code, you can't "call" arbitrary locations in memory. And indirect_call is also runtime typechecked. So you can't call functions with an unexpected type signature. Also (I think) the jump table itself can't be edited by the running wasm module. So there's no way to inject code into the module and run it.

I could be wrong, but I wouldn't be surprised if on balance wasm still ends up safer than native code here. I'm sure there will be more than zero wasm sandbox escapes made by abusing this, but I haven't heard of any so far.

Docs: https://developer.mozilla.org/en-US/docs/WebAssembly/Underst...

- WASM allows writing to 0x0.

You're probably right about this. To be clear, it means if pointers are set to 0 then dereferenced, the program might continue before crashing. And the memory around 0 may be overwritten by an attacker. How bad this is in practice depends on the prevelance of use-after-free bugs (common in C / C++) and what ends up near 0 in memory. In rust, these sort of software bugs seem incredibly rare. And I wouldn't be surprised if wasm compilers for C/C++ start making a memory deadzone here - if they aren't doing that already.

- wasm can easily overflow buffers

Sure, but so can native C code. And unlike native code, wasm can't overflow buffers outside of the data section. So you can't overwrite methods or modify the memory of any other loaded modules. So on net, wasm is still marginally safer than native code here. If you're worried about buffer overflows, use a safer language.

- wasm doesn't have the concept of read-only memory

Interesting! I can see this definitely being useful for system libraries like mmap. This would definitely be nice to have, and it looks like the wasm authors agree with you.

https://github.com/WebAssembly/multi-memory/issues/15

There are stray references to the concept of multiple address spaces (or 'memories') in the wasm spec at present, and I recall at one point you may have always been passing 'memory #0' to your load/store opcodes. It looks like people are still working on that as the solution.

https://github.com/WebAssembly/multi-memory

It's not segmented, so no... or rather, not yet.

The wasm spec already accommodates to some extent the notion of multiple "memories" (i.e. distinct flat heaps), although it only allows for one in practice:

https://webassembly.github.io/spec/core/syntax/modules.html#...

And there's an active proposal to allow for multiple memories:

https://github.com/WebAssembly/multi-memory/blob/main/propos...

In an environment like that, you'd need full-fledged pointers to carry both the memory index and the offset; and then you might want a non-fat "pointer to same memory" alternative for perf. Might as well call them far and near.

This is a slim alternative to Emscripten which focuses only on the C/C++ <=> JS interoperability part:

https://github.com/schellingb/wajic

There's actually a super interesting project called wajic here:

https://github.com/schellingb/wajic

It's basically clang plus wasm-opt and some magic pixie dust which enables some of the most important features of Emscripten, but without the whole 'technology zoo' :)

There's now an interesting alternative to Emscripten called WaJIC:

https://github.com/schellingb/wajic

Enables most of the "Emscripten magic" (like embedding Javascript code into C/C++ files), but in a more bare bones package (apart from clang it essentially just uses the wasm-opt tool from Binaryen for post-processing).

(to be clear, wajic has fewer out-of-the-box features than Emscripten, but it might be an alternative for very small projects which don't need all the compatibility shims which are coming with Emscripten, while still providing tools for calling between C/C++ and JS.

Since I haven't seen it mentioned in the comments yet, here's another interesting project in the general area of "WASM without Emscripten":

https://github.com/schellingb/wajic

This provides an alternative implementation of Emscripten's EM_JS() magic (embed Javascript snippets right in the C/C++ source code), but without the Emscripten SDK. It still needs some additional tools next to Clang, so it sits somewhere between "pure Clang" and "full Emscripten SDK".

Alas, although there is WebAssembly support in the original dos-like, it is still not supported in the bindings for Rust. It would require a Rust toolchain to integrate with WAjic, which I am pretty much unfamiliar with. If you have any idea on how to achieve this, I would love to know.