Meltdown was a single Intel bug, it did not occur on other CPU architectures or on AMD chips. It was a result of asynchronous permission checking and it is a side-channel disclosure (a non-write bug). It is objectively not as bad as the tens of thousands of buffer overruns and memory write vulnerabilities in software.
> 2. Most production-ready UNIXish kernels have had support for running untrusted code (namely BPF bytecode) in the kernel for decades.
Actually it is a security vulnerability as well. In fact, the Project Zero proof of concept for Variant 1 of Spectre was an attack on the BPF interpreter, not even a JIT, it's even worse with a JIT.
> 3. Do you really trust all the code currently running in ring 0 on your computer? In particular, do you trust the executable loader. which handles complex untrusted input? What's the line between "code" and "not code"?
There are levels of trust, of course. I trust the Linux kernel a heck of a lot more than, e.g. V8. And I work on V8. On the WebAssembly implementation. I didn't want to mention it, but yeah, no, I would not put my own code into the kernel.
> 4. On most desktop machines, there's a single user
Again, levels of trust. I would not, e.g. run most userspace software in the kernel, just because it's so broken it will probably bring down the system. All the other things you mention are made easier, not harder by running in the kernel.
> 5. These same machines make a practice of running untrusted, JITted JavaScript and WebAssemy all the time inside the same sandbox you think is too dangerous, and the sandbox works.
I don't want to scare you, but please don't labor under the assumption that web browsers are 100% secure. We have tons of bugs. I mentioned above that the WebAssembly implementation in Chrome is a lot of my work. We've had security vulnerabilities.
> Not perfectly, of course, but also certainly much better than, say, the Linux kernel protects itself from local privilege escalations. Why is the same software sandbox too dangerous for use in kernelspace?
Objectively, no, it isn't better than the Linux kernel. And yes, it is too dangerous. This is based on the hundreds of security bugs that I've been involved with while working on Chrome, and the hundreds more that I wasn't involved with, and the probably hundreds more that are hiding in there. Yes, we take security seriously, and we are very sober about this.
I still think that this is a case of knowing how the sausage is made. I've operated public-facing Linux systems for many years (but I am not either a kernel nor V8 developer, so you know what you're talking about more than I do) and the Linux kernel is ... not good. You take security seriously, and find hundreds of bugs, and it scares you, which is great. The Linux kernel does not (remember the whole "security bugs are just normal bugs" thing, plus the resistance to architectural improvements that kill bug classes, which as far as I can tell you folks seem to be very excited about).
I would hope that you think that V8 + the associated Chrome sandbox (which, to be fair, I think does not have an equivalent in this project) is secure enough to be exposed to random JavaScript / WebAssembly from malicious parties on the internet running and updating 24/7, and keep things reasonably safe, because a billion people do exactly that. I'm not saying it's perfect or unbreakable - I'm just saying I definitely don't trust Linux to be secure against random userspace from malicious parties on the internet running and updating 24/7.
1. Hardware sandboxing (i.e., ring not-0) isn't much better, as seen by Meltdown.
2. Most production-ready UNIXish kernels have had support for running untrusted code (namely BPF bytecode) in the kernel for decades.
3. Do you really trust all the code currently running in ring 0 on your computer? In particular, do you trust the executable loader. which handles complex untrusted input? What's the line between "code" and "not code"?
4. On most desktop machines, there's a single user, and malware being unable to get to ring 0 isn't particularly stymied; it can still exfiltrate your files, stream your webcam, log into your bank, etc. Why is ring 0 more of a concern than untrusted code elsewhere? (In fact, on most Linux desktops, malware can wait until the user runs sudo, inject itself in, and then run insmod and get to ring 0 directly....)
5. These same machines make a practice of running untrusted, JITted JavaScript and WebAssemy all the time inside the same sandbox you think is too dangerous, and the sandbox works. Not perfectly, of course, but also certainly much better than, say, the Linux kernel protects itself from local privilege escalations. Why is the same software sandbox too dangerous for use in kernelspace?