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

[Matthias Welwarsky]

On Samstag, 26. März 2022 05:45:24 CET Dave Hansen wrote:
> On 3/25/22 15:07, Borislav Petkov wrote:
> > 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.
> 
> I'd consider it a bug in the hypervisor, personally. ;)

Of course, no doubt about that.

> But, we do try to make the kernel work even the face of funky
> hypervisors that do things that never occur on real hardware.  If a nice
> patch to fix this up showed up, I'd definitely take a look.

The question is whether a sequence like this could be relevant:

0) CPU announces feature FSRM through cpuid
1) BIOS/firmware disables fast string ops through IA32_MISC_ENABLE before 
loading kernel (for whatever reason)
2) Kernel populates features from cpuid
3) Kernel clears ERMS based on IA32_MISC_ENABLE
4) "alternatives" patching destroys __memmove()

That depends on whether a cpuid instruction after step 1) would still announce 
FSRM. If not, then the whole point is moot and no patch needed (unless we want 
to guard against hypervisor bugs).

-- 
Mit freundlichen Grüßen/Best regards,

Matthias Welwarsky
Project Engineer

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