Messages in this thread | | | Date | Fri, 25 Mar 2022 23:07:19 +0100 | From | Borislav Petkov <> | Subject | Re: x86, possible bug in __memmove() alternatives patching |
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+ dhansen for comment on...
On Fri, Mar 25, 2022 at 09:51:09AM +0100, Matthias Welwarsky wrote: > Hi list, > > I've come across an odd behavior that I'm not sure whether it counts as a bug. > I'm working on an x86 hypervisor at the moment and there was an odd problem > with a linux guest system dying during booting, but only on an intel NUC11 > (i3-1115G4). It booted normally on any other system I tested on. > > The guest was usin kernel version 5.10.89. > > Now, the reason eventually turned out to be __memmove() getting broken when > applying the alternatives. The CPU on the NUC11 has the FSRM feature, which > enables a particular alternative in __memmove(). However, the code assumes > that the ERMS feature is always present with FSRM, otherwise the function gets > broken. Of course this is normally the case, a CPU without ERMS would never > advertise FSRM. > > However: in the function early_intel_setup(), the ERMS feature might get > cleared depending on the IA32_MISC_ENABLE MSR: if fast string operations are > disabled through this MSR, X86_FEATURE_ERMS is removed from the available set, > but X86_FEATURE_FSRM is not removed.
... this thing, whether that's really the case on real hw.
Leaving in the rest for reference.
> Now, the hypervisor I'm working on emulates this MSR and in the particular > case, fast string ops were reported as disabled, leading to the above > mentioned inconsistency in __memmove(). > > The Intel SDM doesn't go into any details if disabling fast string ops through > IA32_MISC_ENABLE will also clear FSRM from cpuid leaf 7, but I think that's > not relevant for the linux kernel anyway because cpuid is parsed before MSRs > are evaluated. > > I know it's is probably a very rare case and Intel recommends having fast > string ops enabled, hence the question: would this be considered a bug in the > kernel that should be fixed? A potential fix could be to clear FSRM together > with ERMS depending on IA32_MISC_ENABLE. > > > Mit freundlichen Grüßen/Best regards, > > Matthias Welwarsky > Project Engineer > > SYSGO GmbH > Office Mainz > Am Pfaffenstein 8 / D-55270 Klein-Winternheim / Germany > Phone: +49-6136-9948-0 / Fax: +49-6136-9948-10 > E-mail: matthias.welwarsky@sysgo.com > _________________________________________________________________________________ > Web: https://www.sysgo.com > Blog: https://www.sysgo.com/blog > Events: https://www.sysgo.com/events > Newsletter: https://www.sysgo.com/newsletter > _________________________________________________________________________________ > Handelsregister/Commercial Registry: HRB Mainz 90 HRB 48884 > Geschäftsführung/Managing Directors: Etienne Butery (CEO), Kai Sablotny (COO) > USt-Id-Nr./VAT-Id-No.: DE 149062328 > > The protection of your personal data is important to us. Under the following > link > you can see the information in accordance with article 13 GDPR: > https://www.sysgo.com/privacy_policy > >
-- Regards/Gruss, Boris.
https://people.kernel.org/tglx/notes-about-netiquette
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