Comment by gpderetta | Hacker News Reader

Comment by gpderetta | Hacker News Reader

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Hi twtw,

I think you were having that discussion with me.

So, I went and read the whole lkml threads you linked and if I understood correctly, regarding spectre v1, the kernel is only expected vulnerable to bpf based attacks or similar. As far as I understand, the speculation barriers are used to protect arrays directly accessible by bpf programs.

There is a mention of out of process attacks to other userspace programs, but no details.

By carefully crafting inputs, I'm ready to admit that it might be theoretical ly possible to attack some exploitable branches, but the big deal with spectre is the high bandwidth that can be attained by directly running code in process.

Do you have any pointer to any description of an even remotely practical out of process spectre v1 attack that doesn't involve executing code in process? Repurposing an interface that is not meant to be used to run code (I e. Build your own VM) is fair game.

Here's alan cox saying what I've been trying to say, from https://marc.info/?l=linux-kernel&m=151503218808512&w=2:

> If you read the papers you need a very specific construct in order to not only cause a speculative load of an address you choose but also to then manage to cause a second operation that in some way reveals bits of data or allows you to ask questions.

> BPF allows you to construct those sequences relatively easily and it's the one case where a user space application can fairly easily place code it wants to execute in the kernel. Without BPF you have to find the right construct in the kernel, prime all the right predictions and measure the result without getting killed off. There are places you can do that but they are not so easy and we don't (at this point) think there are that many.

> The same situation occurs in user space with interpreters and JITs,hence the paper talking about javascript. Any JIT with the ability to do timing is particularly vulnerable to versions of this specific attack because the attacker gets to create the code pattern rather than have to find it.

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> big deal with spectre is the high bandwidth that can be attained by directly running code in process

That depends on your perspective. If you are an OS developer who strives to guarantee process isolation, than it is a pretty big deal that spectre v1 allows you to read memory from the kernel or from other processes, even if it might be tricky to do so. If you write a JS JIT, then yeah you are probably most concerned about the single-process case.

> remotely practical

IMO, most spectre attacks are not remotely practical. No, I don't have a pointer. The only actual demonstrations of spectre I've seen is the one included with the original paper (single process).

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