Re: x86, possible bug in __memmove() alternatives patching

[Borislav Petkov <bp@alien8.de>]

+ 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
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-- 
Regards/Gruss,
    Boris.

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