linux-kernel - Re: [PATCH 4/4] X86: Use KVM CR pin MSRs

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Date:   Thu, 18 Jun 2020 15:49:32 +0000
From:   "Andersen, John" <john.s.andersen@...el.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     corbet@....net, pbonzini@...hat.com, tglx@...utronix.de,
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Subject: Re: [PATCH 4/4] X86: Use KVM CR pin MSRs

On Thu, Jun 18, 2020 at 08:38:06AM -0700, Dave Hansen wrote:
> On 6/18/20 8:26 AM, Andersen, John wrote:
> > On Thu, Jun 18, 2020 at 07:41:04AM -0700, Dave Hansen wrote:
> >>> +config PARAVIRT_CR_PIN
> >>> +       bool "Paravirtual bit pinning for CR0 and CR4"
> >>> +       depends on KVM_GUEST
> >>> +       help
> >>> +         Select this option to have the virtualised guest request that the
> >>> +         hypervisor disallow it from disabling protections set in control
> >>> +         registers. The hypervisor will prevent exploits from disabling
> >>> +         features such as SMEP, SMAP, UMIP, and WP.
> >>
> >> I'm confused.  Does this add support for ""Paravirtual bit pinning", or
> >> actually tell the guest to request pinning by default?
> >>
> >> It says "Select this option to have the virtualised guest request...",
> >> which makes it sound like it affects the default rather than the
> >> availability of the option.
> > 
> > How about this
> > 
> > Select this option to request protection of SMEP, SMAP, UMIP, and WP
> > control register bits when running paravirtualized under KVM. Protection will
> > be active provided the feature is available host side and kexec is disabled via
> > kconfig or the command line for the guest requesting protection.
> 
> It still isn't very clear to me.
> 
> Let's pull the config option out of this patch.  Enable the feature by
> default and do the command-line processing in this patch.
> 
> If you still think a Kconfig option is helpful, add it in a separate
> patch calling out the deficiencies with the boot-time options.

That's right we're going to pull it out anyway and just disable if the
disable_pv_cr_pin command line option is set. Oops. That solves that.

Thank you very much for your review Dave