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Comment by vlovich123 | Hacker News Reader

It’s interesting that it spends a lot of time talking about how memory safe languages don’t help V8 cause of JIT (true) & then talks about a hardening technique that does get helped by Rust. Why not just be honest and say that the switching cost to a new language is too expensive and error prone than play these games? Similarly the discussion about memory tagging - I know that it would harden the security of things like Cloudflare workers. And if most of the exploits are because it’s in-process, why not work on isolating it behind it’s own process (there must be ways to do this securely while not giving up too much performance)?

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…because the overwhelming majority of memory safety bugs in js engines (v8, JSC, and spider monkey) are in operations that would be in unsafe blocks in rust as well?

In multiple decades I can think of a handful of engine bugs that would have been prevented by rust - and those were largely preventable (and now are) in c++ as well.

It is possible for rust to be a safer language than c++ and to also not meaningfully change the security profile of the language.

It’s not just the jit, the interpreter and GCs are also subject largely - necessarily - no more protected by rust than c++.

Did you read the article? It has nothing to do with the JIT. It uses the JIT as a smokescreen to talk about sandbox hardening & the issues within the sandbox are definitely not "unsafe" & 100% mitigated by Rust. Take a look at the relevant quotes I extracted in another comment to draw your attention to what the article is actually talking about (sandbox hardening).

The technique here keeps a large set of objects from escaping the sandbox. Those objects are accessed both by C++ and JIT code. You are right that using Rust instead of C++ would help on the C++ side, but it would not help at all on the JIT code side, and that is by far the major source of exploits.

In other words, even if you write a JS engine in Rust you could benefit greatly from this technique.

I have no issue with the sandbox technique. My critique is that the sandbox escapes just have to find an assumption violation in C++ that leads to a memory violation, meaning that sandbox escapes are easier in V8 due to being written in C++. Having the sandbox & security-relevant runtime pieces written in Rust would do a lot to prevent huge swathes of sandbox escapes more robustly than trying to ensure that all C++ today & that will be written remains hardened.

I don't think it's fair to call them dishonest here. It's pretty clear they've heard about memory safe languages, they've thought about it, they've considered in details the pros and cons.

>why not work on isolating it behind it’s own process (there must be ways to do this securely while not giving up too much performance)?

Well, you make it sound like the easy answer. A good exercise would be to try implementing what you're proposing in a comment. Not necessarily going all the way, but enough to know why it might not be as straightforward as you think.

The people working on V8 are not completely clueless, the concept of moving things out of process or using a memory safe language is not going to be a novel idea that they'll just start working on now that someone clever thought of it.

The dishonest piece is that the first part talks about why Rust doesn't help with the JIT (true) but then really talks about the V8 sandbox & hardening techniques they're applying to it where Rust would help 100%.

> However, assuming these numbers are simply stored as integers somewhere in the JSObject, an attacker could corrupt one of them to break this invariant. Subsequently, the access into the (out-of-sandbox) std::vector would go out of bounds. Adding an explicit bounds check, for example with an SBXCHECK, would fix this.

Or use Rust

> Contrary to the 2nd order vulnerabilities typically found in V8, these sandbox bugs could actually be prevented or mitigated by the approaches discussed earlier. In fact, the particular bug above would already be mitigated today due to Chrome's libc++ hardening

Or use Rust

I'm not saying rewrite the entire thing in Rust, too expensive & would introduce new bugs in the JIT for questionable benefit. But at least mention that & also discuss the technical challenges why the sandbox mechanism isn't written in Rust & what it would take to address those.

Look, I'm not saying the V8 team is making the wrong decisions. My questions are an indication of the shallowness of the blog write-up - why not explain some obvious questions that come up for someone who reads it?

How would rust help you when you're executing jitted code (ie assembly)? The fizzbuzz code would run in rust but the event handler would still be unsafe jitted code.

It doesn't: but it helps with the stuff around it. The article talks about 3 locations for bugs

1) jitted javascript code with subtle bugs due to logic errors in the compiler (Rust's memory safety can't really help here)

2) Bugs in surrounding utility code and the interpreter (Rust can help, but running without a JIT entirely is too slow. Still, it's part of the attack surface either way)

3) Bugs in the sandbox implementation which helps mitigate bugs of the first kind (Rust can help)

AFAIK the main objection raised here is the article dismisses moving to a memory safe language because it doesn't help with 1, but then discusses 2 and 3 where in fact the issues are exactly where memory safety can help.

The whole point is that the sandbox is an approach that can be used in JIT code, where Rust doesn't help.

Take the fizzbuzz example with a missing bounds check. Rust can't prevent you from generating JIT code that omits a bounds check on an array and reads/writes out-of-bounds. The sandbox doesn't prevent out-of-bounds reads/writes, but it guarantees that they will only be able to access data inside the sandbox.

This means that logic bugs in the JIT compiler are no longer immediately exploitable. They must be combined with bugs in the sandbox implementation. The article's claim is that, unlike compiler bugs, sandbox bugs tend to be amenable to standard mitigation techniques.

This article isn't dismissing the value of memory-safe languages. It's identifying a problem space where current memory-safe languages can't help, and providing an alternative solution. Currently, every browser JS engine is written in C++, in part because Rust doesn't solve the big correctness problems. If the sandbox approach works, then using Rust for other parts of the engine becomes more appealing.

To be clear. I have no issue with the sandboxing as a technique. It's perfectly valid and a good idea. My issue is that sandbox escapes are significantly easier than they should be due to the C++ runtime. Rust 100% would help mitigate sandbox escapes more effectively even as the codebase evolves. We know through lots of practical experience that "standard mitigation techniques" for C++ don't actually work all that well (there's at least about an order of magnitude difference in number of exploits possible between that & Rust).

> talks about a hardening technique that does get helped by Rust

What hardening technique discussed in this article would be helped by Rust, and what specific feature of Rust would help?

> This code makes the (reasonable) assumption that the number of properties stored directly in a JSObject must be less than the total number of properties of that object. However, assuming these numbers are simply stored as integers somewhere in the JSObject, an attacker could corrupt one of them to break this invariant. Subsequently, the access into the (out-of-sandbox) std::vector would go out of bounds. Adding an explicit bounds check, for example with an SBXCHECK, would fix this.

> Encouragingly, nearly all "sandbox violations" discovered so far are like this: trivial (1st order) memory corruption bugs such as use-after-frees or out-of-bounds accesses due to lack of a bounds check. Contrary to the 2nd order vulnerabilities typically found in V8, these sandbox bugs could actually be prevented or mitigated by the approaches discussed earlier. In fact, the particular bug above would already be mitigated today due to Chrome's libc++ hardening. As such, the hope is that in the long run, the sandbox becomes a more defensible security boundary than V8 itself

It's still not clear to me what Rust feature would prevent what specific vulnerability here. Rust has bounds-checked and non-bounds-checked array accesses depending on the developer's preference, and so does C++. If there's some point you're making with these quotes you're going to need to simplify it for me since I'm not following.

The defaults are switched though: C++ is unchecked by default, Rust is checked by default.

> Rust has bounds-checked and non-bounds-checked array accesses depending on the developer's preference, and so does C++

You're making it sound like these are the same, the difference is the defaults

Unsafely accessing an element in C++

    vec[i]

Unsafely accessing an element in Rust

    unsafe { vec.get_unchecked(i) }

One of these is screamingly obvious that something potentially unsafe is happening and should be audited more closely, that's the real difference. The cause of potential memory issues is isolated and searchable in `unsafe` blocks rather than being potentially anywhere

So the Rust feature is "screaming obviousness"? Your argument is that the advantage of rewriting the module in Rust is that finding array accesses is visually easier?

Why not just use grep?

I'm not on the V8 team, so can't say why grep didn't find the vulnerabilities. That would perhaps be a good suggestion to make to them!

It's not just that. It's that you can also add `#![forbid(unsafe_code)]` to your crate. Now you know there's 0 potential memory safety issues of any kind.

Keep in mind that failed bounds checks are only 1 memory safety possibility for C++. Another common one is UB behavior which abounds all over the place in C++.

The only language security issue you have to worry about with safe Rust is that integers overflow by default in release mode which means you need to explicitly annotate summations that could cause a security exploit with `saturating_X`, `checked_add`, or `wrapping_add` depending on what behavior you want when you exceed the integer range bounds. In cases where security by default is more important, setting `overflow-checks` to `true` in the Cargo profile is better - that way you have to explicitly opt-in to wrapping behavior if you want more performance in the hot path.

Well that's just false in this context.

    vec[i]

Is safe in v8, blink, jsc, webkit, etc. Rust has a huge number of benefits over c++, but it hurts your argument if you refuse to acknowledge the actual environment the C++ is being used in and make objectively incorrect statements. It implies a lack of understanding of C++ and sounds like all you're doing is parroting other people's critiques without understanding the core issues, which undermines your message.

That said it's still not particularly relevant here, because the issues being presented are bugs in the runtime. e.g. the runtime logic and state results in erroneous behavior. The bugs being discussed are not "you did not use a safe vec" or "you did not use Rc", it's "the size or bounds check in vec is incorrect" or "the ref counting in Rc is incorrect". Rust does not inherently stop those the runtime from having bugs, it simply statically limits where the exposure to unsafe operations can occur.

That's super relevant to program safety, but it's not relevant to safety in the JS VM runtime, where they're performing the operations that would be unsafe{} in rust as well.

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