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I found a bug in Intel Skylake processors

gallium.inria.fr

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Re: I found a bug in Intel Skylake processors

#51

Post by zurn »

userbinator wrote: post #45
hsnewman wrote: post #43

Can this be exploited for malicious code?

It's "unpredictable" what happens, so I think the best you're going to do is a DoS. I.e. if you could get the JS JIT in a browser to generate code like this and execute it repeatedly, you could crash a machine just by visiting a site.

In software systems you can nudge many similar situations to give you control over what badness happens when you drive the system off the rails of the invariants. No reason why things anogous to nop sleds, heap spraying etc would not be applicable here.

Re: I found a bug in Intel Skylake processors

#52

Post by joshuata »

ihnorton wrote: post #13

> I worked from the executable provided by SIOU, first interactively under GDB (but it nearly drove me crazy, as I had to wait sometimes one hour to trigger the crash again), then using a little OCaml script that ran the program 1000 times and saved the core dumps produced at every crash. rr can often be a time-saver in situations by providing deterministic replays up to the point of a crash, whereas coredump analysi…

rr wouldn't help in this case. From the docs, it "emulates a single-core machine. So, parallel programs incur the slowdown of running on a single core." The skylake bug only occurred under heavily threaded loads.

Re: I found a bug in Intel Skylake processors

#53

Post by raverbashing »

acdha wrote: post #49
raverbashing wrote: post #33

Earlier quoted context omitted.

The code produced by Clang is a direct translation of the C code. That's the obvious part of it Most people here are not familiar with x86 assembly and its caveats it seems. Reading the Intel and AMD optimization manuals might be a good start (and yes, the bug is not in GCC it's on the Intel processor)

> Most people here are not familiar with x86 assembly and its caveats it seems. Reading the Intel and AMD optimization manuals might be a good start. Please don't make unsupported assertions that everyone but you is speaking out ignorance. It doesn't add anything to the conversation, especially when dealing with older codebases unless you can prove that this is and never has been the correct way to write that code. O…

Should be easy to prove me wrong if it's as easy as you say, I don't see why you're so annoyed by it

> “CPU optimizations change over time and an open-source project doesn't have a team of experts tracking microbenchmarks to decide when to switch”

Yes, that's what's happening, but the problem comes from the P6 architecture (though Netburst doesn't have those problems), this problem has been known for around 20 years

Re: I found a bug in Intel Skylake processors

#56

Post by piemonkey »

One amusing thing about this epic tale is Serious Industrial OCaml User disregarded direct, relatively easy to implement, very sound advice from Xavier Leroy about how to debug their system! I would like to think that, were I in a similar situation being advised by an expert of that calibre, I would at least humor his suggestions.

Why seek the expert if not for his advice? It brings to mind people disregarding doctors who give them inconvenient medical advice.

Re: I found a bug in Intel Skylake processors

#58

Post by acdha »

pjc50 wrote: post #50
etatoby wrote: post #14

I will be surely downvoted for this, but I would like to remind everyone how this bug is just one of the many consequences of Microsoft's evil policy of encouraging the sale and distribution of proprietary software in executable form. There is no other reason why a 64bit multi-core CPU developed in 2015, that makes heavy use of pipelining and other advanced and complicated code execution strategies, would need to sup…

Shipping software executable form? That ship has sailed, long, long ago. Shipping source is both of little use to the vast majority of non-huge customers, and a way of guaranteeing that your software will be pirated. In fact, the demands of getting paid are moving us more and more to SaaS.

Also, it's just trading one class of problems for another. Rather than supporting a compiled binary you need to support the entire build environment for every customer; having worked at a vendor who did that in the 90s I'm deeply skeptical that it would be an improvement.

Re: I found a bug in Intel Skylake processors

#60

Post by justin66 »

piemonkey wrote: post #56

One amusing thing about this epic tale is Serious Industrial OCaml User disregarded direct, relatively easy to implement, very sound advice from Xavier Leroy about how to debug their system! I would like to think that, were I in a similar situation being advised by an expert of that calibre, I would at least humor his suggestions. Why seek the expert if not for his advice? It brings to mind people disregarding doctor…

I know nothing of OCaml culture or why the author is deemed worthy of having his name italicized, but the doctor comparison is upsetting. If the guy who wrote this:

I was tired of this problem, didn't know how to report those things (Intel doesn't have a public issue tracker like the rest of us), and suspected it was a problem with the specific machines at SIOU (e.g. a batch of flaky chips that got put in the wrong speed bin by accident).

were a doctor, he'd be guilty of malpractice. This bug went unreported eight months longer than it needed to. Am I misreading all this somehow?

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