[HN Gopher] Mitigations=off considered harmful or spurious SIGIL...
___________________________________________________________________
Mitigations=off considered harmful or spurious SIGILL on AMD Zen4
Author : 2bluesc
Score : 86 points
Date : 2023-10-08 17:31 UTC (5 hours ago)
(HTM) web link (forum.level1techs.com)
(TXT) w3m dump (forum.level1techs.com)
| GracielaEdison wrote:
| [dead]
| nightfly wrote:
| Have they tested this on more than one machine? This seems like
| it could be a hardware fault
| 2bluesc wrote:
| 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 where
| mitigations work around it and nobody has noticed.
|
| Seems he had overclocked and maybe disabling the migation has
| revealed an instability in his setup that's otherwise stable (or
| at best marginal).
| tux3 wrote:
| mitigations=off does a lot of things, it's a bundle of options
|
| Perhaps the next step is trying to figure out which mitigation
| exactly causes it to fail. Then kernel peeps should have a
| fighting chance of tracking this down
| simcop2387 wrote:
| In the video comments it looks like he's done some of that
| and it's one of the spectre v2 mitigations. Makes me wonder
| if there's now less QA happening on the oem side without
| mitigations=on.
| AnotherGoodName wrote:
| SMT off fixing it is a good clue. It heavily points to the
| problem being within a core rather than between execution
| cores. That's still a broad area though. Within a core some
| state is inadvertantly shared between the shared threads
| running on it but there's a lot of possibilities of what that
| could be.
| foota wrote:
| Some state being in the CPU or kernel? I guess if you have
| per core state in the kernel that's only accessed by that
| core, then you're safe to modify it without locking, but
| this isn't the case for data shared between hyperthreads of
| a core?
| AnotherGoodName wrote:
| 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 work by making sure to clear everything
| they can when the CPU switches contexts. This includes
| clearing loaded local CPU cache, clearing the branch
| prediction table, clearing all speculative execution
| entries. The mitigations likely avoid a crash by clearing
| something that should be cleared between context switches
| in all cases. As in the mitigations likely accidentally
| fix this. Which is a good clue. It'd be nice if the
| mitigations were more fine grained than all or nothing so
| we could turn on the mitigations one by one but the
| microcode is not open so we can't do this :(.
| yellow_lead wrote:
| He says that he tried stock voltage for everything, and even
| undervolted/underclocked CPU/memory and still had the bug.
| secondcoming wrote:
| 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.
| burnished wrote:
| I think its merely considered gauche, that and people seem to
| have broadly realized anyone could use that phrase to imply
| consensus.
| fabianhjr wrote:
| 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.
| nubinetwork wrote:
| Still waiting for amd to fix random bugs in znver1.
| https://bugs.gentoo.org/724314
| ta988 wrote:
| Unlikely to happen.
| nathants wrote:
| lucky me, i'm running mitigations=off and smt=off on a 7950x.
| zamadatix wrote:
| Out of curiosity did you have smt=off for performance reasons
| or other reasons?
| nathants wrote:
| performance testing vulkan and physx on linux.
| [deleted]
| Ckirby wrote:
| Since when is level1techs taken seriously as a source?
|
| A bunch of YouTube Kids with an unwarranted sense of self
| importance found this "major" "huge" case of nothing
| ezekiel68 wrote:
| You know how it is in social media -- just like in academia:
| "Publish or perish".
| sph wrote:
| Someone got an axe to grind.
___________________________________________________________________
(page generated 2023-10-08 23:00 UTC)