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

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Date:   Sat, 26 Mar 2022 09:27:55 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Matthias Welwarsky <matthias.welwarsky@...go.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        linux-kernel@...r.kernel.org, x86-ml <x86@...nel.org>
Subject: Re: x86, possible bug in __memmove() alternatives patching

On Fri, Mar 25, 2022 at 09:45:24PM -0700, 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. ;)

That's a given.

> 
> 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.

So, more to the point, it is about this chunk:

        /*
         * If fast string is not enabled in IA32_MISC_ENABLE for any reason,
         * clear the fast string and enhanced fast string CPU capabilities.
         */
        if (c->x86 > 6 || (c->x86 == 6 && c->x86_model >= 0xd)) {
                rdmsrl(MSR_IA32_MISC_ENABLE, misc_enable);
                if (!(misc_enable & MSR_IA32_MISC_ENABLE_FAST_STRING)) {
                        pr_info("Disabled fast string operations\n");
                        setup_clear_cpu_cap(X86_FEATURE_REP_GOOD);
                        setup_clear_cpu_cap(X86_FEATURE_ERMS);
                }
        }

we already check whether fast strings was disabled, regardless of HV or
not. Question is, should we clear X86_FEATURE_FSRM there too. I wanna
say yes.

Or is it that, *if* MSR_IA32_MISC_ENABLE_FAST_STRING is clear, the FSRM
CPUID bit was not set either so nothing to clear...

Hmm?

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette