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Encryption-breaking, password-leaking bug in many AMD CPUs could take months to fix (arstechnica.com)
submitted 11 months ago by [email protected] to c/[email protected]
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The bug allows attackers to swipe data from a CPU's registers. [...] the exploit doesn't require physical hardware access and can be triggered by loading JavaScript on a malicious website.

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[-] [email protected] 1 points 11 months ago

I’m curious - does this kind of report make people less likely to go with an AMD cpu? The last time I was thinking about building a new pc, AMD had just definitively taken the lead in speed per dollar, and I would have gone with one of the higher end chips. I’m not sure whether this would have affected my decision, but I’d probably be concerned with performance degradation as well as the security issue. I’d have waited for the patch to buy a system with updated firmware, but Od still want to see what the impact was as well as learn more about the exploit and whether there were additional issues.

I ended up just getting a steam deck and all of my other computers are macs, so it’s hard to put myself back into the builder’s/buyer’s headspace.

[-] [email protected] 1 points 11 months ago

Honestly no. Remember Spectre & Meltdown vulnerabilities back in 2018? Yeah that security bug that only affected Intel CPUs until it was patched seriously told consumers and enterprise customers to “please turn off hyperthreading” to prevent exposure. Fucking LOL. Voluntarily cut my CPU performance in half!? Based on a theoretical exploit that was only found in a very specific and controlled environment before everyone started FREAKING out?

[-] [email protected] 1 points 11 months ago* (last edited 11 months ago)

Neither spectre nor meltdown are specific to Intel. They may have been discovered on Intel hardware but the same attacks work against any system with branch prediction or load speculation. The security flaw is inherent to those techniques. We can mitigate them with better address space separation and address layout randomization. That is, we can prevent one process from reading another process's data (which was possible with the original attacks), but we can't guarantee a way to prevent malicious browser tab from reading data from a different tab (for example), even if they are both sandboxed. We also have some pretty cool ways to detect it using on-chip neural networks, which is a very fancy mitigation. Once it's detected, a countermeasure can start screwing with the side channel to prevent leakage at a temporary performance cost.

Also, disabling hyper threading won't cut your performance in half. If the programs that are running can keep the processor backend saturated, it wouldn't make any noticeable difference. Most programs can only maintain about 70-80% saturation, and hyper threading fills in the gaps. However the result is that intensive, inherently parallelizable programs are actually penalized by hyper threading, which is why you occasionally see advice to disable it from people who are trying to squeeze performance out of gaming systems. For someone maintaining a server with critically sensitive data, that was probably good advice. For your home PC, which is low risk... you're probably not worried about exposure in the first place. If you have a Linux computer you can probably even disable the default mitigations if you wanted.

[-] [email protected] 1 points 11 months ago* (last edited 11 months ago)

does this kind of report make people less likely to go with an AMD cpu?

I doubt it, since Intel has also had its share of similar CPU security issues. For several generations the understanding has been that Intel's CPUs arrive with impressive performance on day 1, then gradually leak that performance away as security issues are patched over subsequent months.

[-] [email protected] 1 points 11 months ago* (last edited 11 months ago)

I think the mitigations are acceptable, but for people who don't want to worry about that, yes, it could put them off choosing AMD.

To reiterate what Tavis Ormandy (who found the bug) and other hardware engineers/enthusiasts say, getting these things right is very hard. Modern CPUs apply tons of tricks and techniques to go fast, and some of them are so beneficial that we accept that they lead to security risks (see Spectre and Hertzbleed for example). We can fully disable those features if needed, but the performance cost can be extreme. In this case, the cost is not so huge.

Plus, even if someone were to attack your home computer specifically, they'd have to know how to interpret the garbage data that they are reading. Sure, there might be an encryption key in there, but they'd have to know where (and when) to look*. Indeed, mitigations for attacks like spectre and hertzbleed typically include address space randomization, so that an attacker can't know exactly where to look.

With Zenbleed, the problem is caused by something relatively simple, which amounts to a use-after-free of an internal processor resource. The recommended mitigation at the moment is to set a "chicken bit," which makes the processor "chicken out" of the optimization that allocates that resource in the first place. I took a look at one of AMD's manuals and I'd guess for most code, setting the chicken bit will have almost no impact. For some floating-point heavy code, it could potentially be major, but not catastrophic. I'm simplifying by ignoring the specifics but the concept is actually entirely accurate.

* If they are attacking a specific encrypted channel, they can just try every value they read, but this requires the attack to be targeted at you specifically. This is obviously more important for server maintainers than for someone buying a processor for their new gaming PC.

[-] [email protected] 1 points 11 months ago

What are the rules on responsible disclosure? Shouldnt they have waited until patches are ready before public disclosure of the exploit?

[-] [email protected] 2 points 11 months ago

I mean, this was disclosed to AMD a few months back and there actually is a patch available currently for Epyc CPUs.

It'd be nice if they waited until all the patches were out, but I'd rather this than a full zero-day exploit of this scale in the wild.

[-] [email protected] 1 points 11 months ago* (last edited 11 months ago)

It's very weird it takes them so long to fix this for consumers tbh. You'd think they could just take the snippet from Epyc and patch it into AGESA, since it's exactly the same architecture. December is hardly acceptable for a critical vulnerability like this.

This is a great opportunity to remind people that NoScript, HTTPS-only modes and filter lists for malicious websites (to use in your adblock of choice) exist. Use them.

[-] [email protected] 1 points 11 months ago

This kind of shit is exactly why I use uMatrix as well as uBlock Origin. It allows me to monitor and control 3rd party scripts and allow only what's needed for a website. If a malicious 3rd party script does happen to get injected into things; I usually notice...especially if it actually breaks shit on the website by not loading it.

this post was submitted on 26 Jul 2023
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