Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

Mitigations=off considered harmful or spurious SIGILL on AMD Zen4

forum.level1techs.com

Post reply on HN
Print view View original

31–40 of 47 posts

Re: Mitigations=off considered harmful or spurious SIGILL on AMD Zen4

#31

Post by AnotherGoodName »

vlovich123 wrote: post #23
AnotherGoodName wrote: post #19

Earlier quoted context omitted.

Within the CPU core (terminology here gets awkward). Spectre in its simplest to exploit form used the fact that the branch prediction based state wasn't completely cleared between running different threads on the CPU and led to a really easy timing attack. Time your own code to see which way the branch of the previous code likely went. There's a lot of CVE's around this but that's the simplest case. The mitigations w…

Is it likely the issue is within the Linux kernel itself or within the microcode for the CPU?

I'm going to say microcode until someone reproduces on a completely different architecture :).

Re: Mitigations=off considered harmful or spurious SIGILL on AMD Zen4

#32

Post by [deleted] »

fabianhjr wrote: post #8
secondcoming wrote: post #5

Isn't there a rule about titles containing 'considered harmful'? Either way, this is pretty interesting and the guy is pretty excited about finding it.

There is a rule on mostly not editorializing titles and in this case that is the title of the source article. > Otherwise please use the original title, unless it is misleading or linkbait; don't editorialize.

[deleted]

Re: Mitigations=off considered harmful or spurious SIGILL on AMD Zen4

#33

Post by eigenform »

would've been nice to see a whole copy of the actual GCC binary in question :(

edit: looks like it dies in this: https://github.com/gcc-mirror/gcc/blob/master/gcc/tree-switc...

edit2: it also would've been nice to see the actual bytes being decoded when it faults, maybe it ends up spuriously writing over .text

edit3: on the LKML https://lore.kernel.org/lkml/D99589F4-BC5D-430B-87B2-72C2037...

Re: Mitigations=off considered harmful or spurious SIGILL on AMD Zen4

#34

Post by jdiff »

AnotherGoodName wrote: post #31
vlovich123 wrote: post #23

Earlier quoted context omitted.

Is it likely the issue is within the Linux kernel itself or within the microcode for the CPU?

I'm going to say microcode until someone reproduces on a completely different architecture :).

A lot of these mitigations are joint efforts between the kernel and CPU, and the mitigations kernel parameter isn't a binary on/off, you can configure individual mitigations. It's not a single flag that does signals the microcode to do spooky proprietary things.

Re: Mitigations=off considered harmful or spurious SIGILL on AMD Zen4

#35

Post by mastensg »

Levitating wrote: post #28
2bluesc wrote: post #2

Interesting walk through in the linked video as he tried to troubleshoot this: https://www.youtube.com/live/1UnoBfw6soI Pretty shocking to see (extremely unlikely) non-malicious code work / not-work depending on a security mitigation setting. Curious to see where this goes as to whether it's a kernel bug and nobody is paying attention to `mitigations=off` now or the unlikely outcome that it's an actual hardware bug w…

What does he say at 8:17?[1] Arsink Overism? [1]: https://www.youtube.com/live/1UnoBfw6soI?si=R8nJ1FxdE4zBuO0i...

"rsync algorithm"

Re: Mitigations=off considered harmful or spurious SIGILL on AMD Zen4

#36

Post by ta988 »

vlovich123 wrote: post #24
ta988 wrote: post #11

Earlier quoted context omitted.

Unlikely to happen.

From the end of the bug tracker sounds like the issue is actually resolved (or at least no longer repros)

yes solved in the compilers not at the cpu level.

Re: Mitigations=off considered harmful or spurious SIGILL on AMD Zen4

#37

Post by iforgotpassword »

I think there was a post a few months ago that in recent AMD CPUs with up to date microcode, mitigations=off actually runs slower than leaving them on. This here seems to be the final nail in the coffin for turning them off, at least on AMD. In the very beginning we decided to turn them off on our fleet of Linux devices, but stopped after that post, even though we don't even run any AMD currently.

Re: Mitigations=off considered harmful or spurious SIGILL on AMD Zen4

#40

Post by [deleted] »

tyrfing wrote: post #26
2bluesc wrote: post #2

Interesting walk through in the linked video as he tried to troubleshoot this: https://www.youtube.com/live/1UnoBfw6soI Pretty shocking to see (extremely unlikely) non-malicious code work / not-work depending on a security mitigation setting. Curious to see where this goes as to whether it's a kernel bug and nobody is paying attention to `mitigations=off` now or the unlikely outcome that it's an actual hardware bug w…

Mitigations can have very large effects on system stability, I ran into this with my old Haswell system. Mitigations on allowed, I believe, 100Mhz higher at lower voltage - but it may have been 200Mhz. These settings were tested over many months, completely stable, and mitigations off with the same setting wouldn't even allow booting. Huge effect relative to anything else, basically like adding an extra 0.1V vcore.

[deleted]
Post reply on HN