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ZombieLoad: Cross Privilege-Boundary Data Leakage on Intel CPUs

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Re: ZombieLoad: Cross Privilege-Boundary Data Leakage on Intel CPUs

#101

Post by dpwm »

cfallin wrote: post #88
Fej wrote: post #76

What is the recommended course of action? Stop buying Intel products, and devices which contain them? What about devices with older processors? I'm still running a Sandy Bridge rig and it works fine, except for the side channel vulnerablities. It's probably not going to be patched. I also have a cheaper computer with a Skylake processor, which is newer yet still vulnerable! It's only a matter of time until something…

The stream of critical CPU vulnerabilities starting with Spectre/Meltdown last year are related to speculative execution , not just Intel. (AMD and ARM CPUs are also vulnerable to Spectre, for example.) Intel CPUs are sometimes vulnerable to additional attacks because they speculate in more scenarios than other designs. But fundamentally, as long as multiple different trust domains are sharing one CPU that speculates…

> Spectre/Meltdown, which leak the speculative information via the cache side-channel, completely blindsided the architecture community; it wasn't just one incompetent company.

In the x86 space, Meltdown absolutely was down to one company apparently deciding to over-optimize for performance.

I can't find it now, but I remember reading a thread from (I think) the OpenBSD devs about how the Intel MMU documentation described fairly sane behaviours and how far the reality deviated from the documentation.

Re: ZombieLoad: Cross Privilege-Boundary Data Leakage on Intel CPUs

#102

Post by ccozan »

School-Cotton wrote: post #81
pcwalton wrote: post #73

Earlier quoted context omitted.

You can kiss any semblance of reasonable performance goodbye if you eliminate "speculative execution". Pipelining is the most basic tool in the toolbox. Even microcontrollers do it.

To give ballpark numbers: modern Intel processors can retire a few instructions per cycle in tight loops (4 is the theoretical maximum; > 2 is realistic in a lot of high-performance code). A branch misprediction wastes 10-15 cycles. So getting rid of speculation entirely, and stalling on every branch, would waste time equivalent to dozens of instructions. On typical code that has a branch every few instructions, this…

Can we actually compute without branching? Genuine question.

What architecture would do that, and how?

Re: ZombieLoad: Cross Privilege-Boundary Data Leakage on Intel CPUs

#103

Post by strmpnk »

bin0 wrote: post #93
Fej wrote: post #76

What is the recommended course of action? Stop buying Intel products, and devices which contain them? What about devices with older processors? I'm still running a Sandy Bridge rig and it works fine, except for the side channel vulnerablities. It's probably not going to be patched. I also have a cheaper computer with a Skylake processor, which is newer yet still vulnerable! It's only a matter of time until something…

From what I have heard, these flaws mostly affect Intel because they are the largest CPU manufacturer. They also dominate the datacenter and cloud-compute industries for now, which are by far the highest value targets. But as an Intel consumer, I am not happy. My understanding is that more stuff can be fixed in microcode, but I suppose a bug could show up which was not practically fixable. If that happened, I would c…

> these flaws mostly affect Intel because they are the largest CPU manufacturer

It doesn't have anything do to with how large Intel is. They have clearly made a more aggressive hardware design which has more corner cases to break. The designs are broken and microcode can patch some variants of these side channels but the overhead is becoming a problem.

In this case it's not certain if microcode can address the problem but if it can't, disabling of SMT (hyperthreading) can be a significant cost for some workloads (well above 10% for things that haven't been specifically tuned to avoid cache misses, which is most software in my experience).

Re: ZombieLoad: Cross Privilege-Boundary Data Leakage on Intel CPUs

#105

Post by lallysingh »

IgorPartola wrote: post #84

So at what point do we start producing CPUs specifically aimed at running a kernel/userland? Why don't we have a CPU architecture where a master core is dedicated to running the kernel and a bunch of other cores run userland programs? I am genuinely curious. I understand that x86 is now the dominant platform in cloud computing. But it's not like virtualization needs to be infinitely nested, right? Why not have the ho…

The hit is in IPC between the kernel and userland processes. If you really want to pay it, then just go microkernel. You can do that today.

Re: ZombieLoad: Cross Privilege-Boundary Data Leakage on Intel CPUs

#106

Post by yalok »

> macOS performance: Testing conducted by Apple in May 2019 showed as much as a 40% reduction in performance with tests that include multithreaded workloads and public benchmarks. Performance tests are conducted using specific Mac computers. Actual results will vary based on model, configuration, usage, and other factors.

from here: https://support.apple.com/en-us/HT210107

Re: ZombieLoad: Cross Privilege-Boundary Data Leakage on Intel CPUs

#107

Post by Symmetry »

jupp0r wrote: post #96
Fej wrote: post #94

Earlier quoted context omitted.

Yes, the vulnerabilities are not just Intel, but they're mostly limited to Intel CPUs. Why is AMD less prone to these mistakes? Perhaps there are simply fewer researchers looking into AMD processors?

The focus is on Intel partially because it's the most valuable target, so we might be looking at selection bias.

That's really not the case. Once you know of an attack of this sort it's pretty easy to test everyone's chips for it. Bascially every Intel chip since Nehalem is vulnerable to Meltdown, as are IBM's POWER chips, as is one out-of-order ARM core but not any of the others. And we know that AMD chips are safe from that vulnerability.

Whether you run security checks in sequence or in parallel with a data access is pretty fundamental to the design of a core. Doing the later is has performance advantages but it's really hard to verify that won't be any architectural leaks as a result, let alone these micro-architectural leaks that nobody was thinking about.

Re: ZombieLoad: Cross Privilege-Boundary Data Leakage on Intel CPUs

#108

Post by strmpnk »

INTPenis wrote: post #104

So I'd love to post an Ask HN: Which AMD Laptops would you recommend for work, alternatives to Thinkpads? I've noticed some Thinkpads with AMD CPUs but I feel like I'm on virgin ground when it comes to AMD and their integrated GPU offerings.

I've been eyeing more release details on the ThinkPad X395 which was recently announced. "Coming Soon" is probably means early June for some select configurations. I think these will fit in the premium/professional laptop space better than some of the bargain laptops that carried AMD chips in the past.

I believe others OEMs are developing similar offerings as well but I can't find any quick links for newer SKUs like the Ryzen 7 3700U which offers the improved Zen+ revisions which will specifically improve battery life and heat issues.

Re: ZombieLoad: Cross Privilege-Boundary Data Leakage on Intel CPUs

#109

Post by pcwalton »

ccozan wrote: post #102
School-Cotton wrote: post #81

Earlier quoted context omitted.

To give ballpark numbers: modern Intel processors can retire a few instructions per cycle in tight loops (4 is the theoretical maximum; > 2 is realistic in a lot of high-performance code). A branch misprediction wastes 10-15 cycles. So getting rid of speculation entirely, and stalling on every branch, would waste time equivalent to dozens of instructions. On typical code that has a branch every few instructions, this…

Can we actually compute without branching? Genuine question. What architecture would do that, and how?

Old GPUs did that. It wasn't very pleasant to program with. :)

Re: ZombieLoad: Cross Privilege-Boundary Data Leakage on Intel CPUs

#110

Post by makomk »

cfallin wrote: post #88
Fej wrote: post #76

What is the recommended course of action? Stop buying Intel products, and devices which contain them? What about devices with older processors? I'm still running a Sandy Bridge rig and it works fine, except for the side channel vulnerablities. It's probably not going to be patched. I also have a cheaper computer with a Skylake processor, which is newer yet still vulnerable! It's only a matter of time until something…

The stream of critical CPU vulnerabilities starting with Spectre/Meltdown last year are related to speculative execution , not just Intel. (AMD and ARM CPUs are also vulnerable to Spectre, for example.) Intel CPUs are sometimes vulnerable to additional attacks because they speculate in more scenarios than other designs. But fundamentally, as long as multiple different trust domains are sharing one CPU that speculates…

That's not exactly true. Broadly speaking, there have been two very different kinds of speculative execution vulnerabilities with different security implications and workarounds. Spectre and its relatives are an attack on trusted code that process untrusted data using certain code patterns guarded by conditionals that can be speculatively executed; they're inherent to speculative execution past branches, but they require specific code patterns that can be avoided to work around the issue.

These vulnerabilites and Meltdown allow untrusted code to speculatively access data that it shouldn't be allowed to access at all, and use that speculative access to leak data itself. Unlike Spectre this can be (and to some extent has to be) fixed at the hardware level, because the hardware itself is failing to protect sensitive data. This class of vulnerability seems to have been mostly Intel-exclusive so far (with the main exception being one unreleased ARM chip that was vulnerable to Meltdown). There's nothing inherent about modern high-performance CPUs that requires them to be designed this way.

Edit: This slipped my mind, but Foreshadow / Level 1 Terminal Fault was yet another similar Intel-only processor vulnerability that allowed speculative access to data the current process should not be able to access. It's definitely a pattern.

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