linux-kernel - Re: [PATCH v4 70/75] x86/head/64: Don't call verify

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Date:   Wed, 15 Jul 2020 17:48:56 +0200
From:   Joerg Roedel <jroedel@...e.de>
To:     Kees Cook <keescook@...omium.org>
Cc:     Joerg Roedel <joro@...tes.org>, x86@...nel.org, hpa@...or.com,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Jiri Slaby <jslaby@...e.cz>,
        Dan Williams <dan.j.williams@...el.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Juergen Gross <jgross@...e.com>,
        David Rientjes <rientjes@...gle.com>,
        Cfir Cohen <cfir@...gle.com>,
        Erdem Aktas <erdemaktas@...gle.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Mike Stunes <mstunes@...are.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Martin Radev <martin.b.radev@...il.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH v4 70/75] x86/head/64: Don't call verify_cpu() on
 starting APs

Hi Kees,

as a general note: With SEV-ES the guest kernel will get #VC exceptions
for events that, without SEV-ES, would just cause a #VMEXIT to the
hypervisor.

On Wed, Jul 15, 2020 at 08:26:14AM -0700, Kees Cook wrote:
> On Wed, Jul 15, 2020 at 11:26:38AM +0200, Joerg Roedel wrote:
> > That MSR is Intel-only, right? The boot-path installed here is only used
> > for SEV-ES guests, running on AMD systems, so this MSR is not even
> > accessed during boot on those VMs.
> 
> Oh, hrm, yes, that's true. If other x86 maintainers are comfortable with
> this, then okay. My sense is that changing the early CPU startup paths
> will cause trouble down the line.

The AP startup path does not change for non SEV-ES guests. But under
SEV-ES everything that might cause a #VC exception must be avoided until
the kernel is ready to handle them. With the current patches this
happens when the AP runs in 64bit long-mode and loaded TSS and IDT.
Therefore a slightly different AP boot-path is needed for SEV-ES guests.

> So, going back to the requirements here ... what things in verify_cpu()
> can cause exceptions? AFAICT, cpuid is safely handled (i.e. it is
> detected and only run in a way to avoid exceptions and the MSR
> reads/writes are similarly bound by CPU family/id range checks). I must
> be missing something. :)

It is actually the CPUID instructions that cause #VC exceptions. The
MSRs that are accessed on AMD processors are not intercepted in the
hypervisors this code has been tested on, so these will not cause #VC
exceptions.

Regards,

	Joerg