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

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Date:   Wed, 15 Jul 2020 11:26:38 +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,

thanks for your reviews!

On Tue, Jul 14, 2020 at 06:40:30PM -0700, Kees Cook wrote:
> Eek, no. MSR_IA32_MISC_ENABLE_XD_DISABLE needs to be cleared very early
> during CPU startup; this can't just be skipped.

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.

The alternative is to set up exception handling prior to calling
verify_cpu, including segments, stack and IDT. Given that verify_cpu()
does not add much value to SEV-ES guests, I'd like to avoid adding this
complexity.

> Also, is UNWIND_HINT_EMPTY needed for the new target?

Yes, I think it is, will add it in the next version.

Regards,

	Joerg