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

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Date:   Wed, 31 May 2023 11:14:52 +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,
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        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,
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        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 08:52:32PM +0200, Peter Zijlstra wrote:

> > 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.

Tom; could you please investigate if this can be enforced in ucode?

Ideally #HV would have an internal latch such that a recursive #HV will
terminate the guest (much like double #MC and tripple-fault).

But unlike the #MC trainwreck, can we please not leave a glaring hole in
this latch and use a spare bit in the IRET frame please?

So have #HV delivery:
 - check internal latch; if set, terminate machine
 - set latch
 - write IRET frame with magic bit set

have IRET:
 - check magic bit and reset #HV latch