"Speculative instruction fetch to device memory crashes device" is in my list of favorite bugs ever found. I had ran into it on a Cortex-M4 without an MPU, so I couldn't even fix the memory attributes, the only solution was to gerrymander the compiled code to prevent the speculative fetch.

It's crazy that Armv8/9-A permits speculative instruction fetches to device memory. Crazy enough that I had to look it up to believe it:

"Hardware does not prevent speculative instruction fetches from a memory location with any of the Device memory attributes unless the memory location is also marked as execute-never for all Exception levels." - ARM DDI 0487K.a § B2.15.2

The Armv7-M spec is less clear. It does say: "The architecture does not permit speculative accesses to memory marked as Device," in contrast to Armv8/9-A which qualifies a similar statement with "data accesses". But then it later says "To ensure correctness, read-sensitive locations must be marked as non-executable". (This is all from ARM DDI 0403E.e § A3.5.7)

I don't have v7-A older handy to compare what they say.

On this theme of it not being just about security: if you have a bug like a use after free and it happens to cover a function pointer, the nx bit can ensure that when you follow that pointer through a call, you get a clean trap as close to the failure point as possible. If it blindly executed stale bytes as code, maybe the crash and stack trace doesn't look as nice.

But then, a lot of correctness bugs like that are also security problems.

Nowadays, it's considered bad form to map anything at 0... you now know one of the reasons why!

In the case that I know that has a 1:1 identity mapping of the address space, the physical address space doesn't actually have anything at 0 either, so it still doesn't need to be mapped.

I appreciated this article. I've asked our CSP guy the difference, but this helps me better understand device vs ns. I also appreciate that this doesn't appear AI written.

This is doubly weird because actually purposefully executing from device memory is not allowed: “Trying to execute code from a region marked as Device is UNPREDICTABLE.” So: can’t reliably execute from there but can speculatively instruction fetch from there. Funsies!

https://support.arm.com/documentation/100941/0101/Memory-typ...

I really wish I understood this, it hits a bunch of topics that I've heard of and are/sound interesting, but I don't know enough to follow it. I don't get the link between the NX bit (which I get) and speculative access.

Honestly feels like a misdesign in ARM. Where does it ever make sense for Device memory to not be data prefetchable, but allow instruction prefetch? It should IMHO disable all prefetch…

> while ARM does provide a reference implementation of the architecture, vendors are free to customize it at will, or even roll their own implementations

A nitpick, but this is only true for some vendors, depending on their license, and is very much company-by-company. Many vendors, even big names like Meta, don't have the ability to roll their own. And even for the ones who do, 'customize at will' is a bit strong, as ARM very much does want to maintain uniformity across userspace implementations. E.g. Nvidia shouldn't add new traps for architecturally-legal behavior, since then code compiled for Apple hardware wouldn't work on Grace. Or worse, not trap for architecturally-illegal behavior, since then code compiled for Grace might not work for anyone else at all!

[dead]

this is so amazing

Hot take: NX bit is shit W^X is shit. proper JIT is having objects written as needed, and cache line flushing is full bullshit, we need self modifying code as a first class citizen and with modern techiques we can have it work and not be crazy slow, it is currently cuz shits fucked, but we can do better.