linux-kernel - Re: [PATCH] x86/sev: Check whether SEV or SME is supported first

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Date:   Tue, 1 Jun 2021 16:14:22 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Pu Wen <puwen@...on.cn>, Joerg Roedel <jroedel@...e.de>,
        x86@...nel.org, joro@...tes.org, thomas.lendacky@....com,
        dave.hansen@...ux.intel.com, peterz@...radead.org,
        tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
        sashal@...nel.org, gregkh@...uxfoundation.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        stable@...r.kernel.org
Subject: Re: [PATCH] x86/sev: Check whether SEV or SME is supported first

On Tue, Jun 01, 2021, Borislav Petkov wrote:
> On Mon, May 31, 2021 at 10:56:50PM +0800, Pu Wen wrote:
> > Thanks for your suggestion, I'll try to set up early #GP handler to fix
> > the problem.
> 
> Why? AFAICT, you only need to return early in sme_enable() if CPUID is
> not "AuthenticAMD". Just do that please.

I don't think that would suffice, presumably MSR_AMD64_SEV doesn't exist on older
AMD CPUs either.  E.g. there's no mention of MSR 0xC001_0131 in the dev's guide
from 2015[*].

I also don't see the point in checking the vendor string.  A malicious hypervisor
can lie about CPUID.0x0 just as easily as it can lie about CPUID.0x8000001f, so
for SEV the options are to either trust the hypervisor or eat #GPs on RDMSR for
non-SEV CPUs.  If we go with "trust the hypervisor", then the original patch of
hoisting the CPUID.0x8000001f check up is simpler than checking the vendor string.

[*] https://www.amd.com/system/files/TechDocs/48751_16h_bkdg.pdf