linux-kernel - Re: [RFC PATCH V6 01/14] x86/sev: Add a #HV exception handler

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Message-ID: <20230530185232.GA211927@hirez.programming.kicks-ass.net>
Date:   Tue, 30 May 2023 20:52:32 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Tom Lendacky <thomas.lendacky@....com>
Cc:     "Gupta, Pankaj" <pankaj.gupta@....com>,
        Tianyu Lan <ltykernel@...il.com>, luto@...nel.org,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
        seanjc@...gle.com, pbonzini@...hat.com, jgross@...e.com,
        tiala@...rosoft.com, kirill@...temov.name,
        jiangshan.ljs@...group.com, ashish.kalra@....com,
        srutherford@...gle.com, akpm@...ux-foundation.org,
        anshuman.khandual@....com, pawan.kumar.gupta@...ux.intel.com,
        adrian.hunter@...el.com, daniel.sneddon@...ux.intel.com,
        alexander.shishkin@...ux.intel.com, sandipan.das@....com,
        ray.huang@....com, brijesh.singh@....com, michael.roth@....com,
        venu.busireddy@...cle.com, sterritt@...gle.com,
        tony.luck@...el.com, samitolvanen@...gle.com, fenghua.yu@...el.com,
        pangupta@....com, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, linux-hyperv@...r.kernel.org,
        linux-arch@...r.kernel.org
Subject: Re: [RFC PATCH V6 01/14] x86/sev: Add a #HV exception handler

On Tue, May 30, 2023 at 10:59:01AM -0500, Tom Lendacky wrote:
> On 5/30/23 09:35, Peter Zijlstra wrote:
> > On Tue, May 30, 2023 at 02:16:55PM +0200, Gupta, Pankaj wrote:
> > > 
> > > > > Add a #HV exception handler that uses IST stack.
> > > > > 
> > > > 
> > > > Urgh.. that is entirely insufficient. Like it doesn't even begin to
> > > > start to cover things.
> > > > 
> > > > The whole existing VC IST stack abuse is already a nightmare and you're
> > > > duplicating that.. without any explanation for why this would be needed
> > > > and how it is correct.
> > > > 
> > > > Please try again.
> > > 
> > > #HV handler handles both #NMI & #MCE in the guest and nested #HV is never
> > > raised by the hypervisor.
> > 
> > I thought all this confidental computing nonsense was about not trusting
> > the hypervisor, so how come we're now relying on the hypervisor being
> > sane?
> 
> That should really say that a nested #HV should never be raised by the
> hypervisor, but if it is, then the guest should detect that and
> self-terminate knowing that the hypervisor is possibly being malicious.

I've yet to see code that can do that reliably.