linux-kernel - Re: [PATCH] KVM: X86: Allow userspace to define the microcode version

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Message-ID: <20180226104921.GA4377@pd.tnic>
Date:   Mon, 26 Feb 2018 11:49:21 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH] KVM: X86: Allow userspace to define the microcode version

On Mon, Feb 26, 2018 at 06:06:42PM +0800, Wanpeng Li wrote:
> I think it is the host admin(e.g. cloud provider)'s responsibility to
> set an expected microcode revision.

+       vcpu->arch.microcode_version = 0x1;

That already looks pretty arbitrary and non-sensical to me.

>In addition, the non-sensical value which is written by the guest will
>not reflect to guest-visible microcode revision and just be ignored in
>this implementation.

Huh? How so?

So a guest will have *two* microcode revisions - both of which are most
likely wrong?!

This whole thing sounds like the wrong approach to me.

> Linux (among the others) has checks to make sure that certain features
> aren't enabled on a certain family/model/stepping if the microcode version
> isn't greater than or equal to a known good version.

It sounds to me like the proper fix is to make the kernel *not* look at
microcode revisions when running virtualized. The same way we're not
loading microcode in a guest:

        if (native_cpuid_ecx(1) & BIT(31))

Letting userspace control the microcode revision number is revision
number management SNAFU waiting to happen IMO.

-- 
Regards/Gruss,
    Boris.

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