Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Tue, 30 May 2023 20:52:32 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Tom Lendacky <thomas.lendacky@amd.com>
Cc:     "Gupta, Pankaj" <pankaj.gupta@amd.com>,
        Tianyu Lan <ltykernel@gmail.com>, luto@kernel.org,
        tglx@linutronix.de, mingo@redhat.com, bp@alien8.de,
        dave.hansen@linux.intel.com, x86@kernel.org, hpa@zytor.com,
        seanjc@google.com, pbonzini@redhat.com, jgross@suse.com,
        tiala@microsoft.com, kirill@shutemov.name,
        jiangshan.ljs@antgroup.com, ashish.kalra@amd.com,
        srutherford@google.com, akpm@linux-foundation.org,
        anshuman.khandual@arm.com, pawan.kumar.gupta@linux.intel.com,
        adrian.hunter@intel.com, daniel.sneddon@linux.intel.com,
        alexander.shishkin@linux.intel.com, sandipan.das@amd.com,
        ray.huang@amd.com, brijesh.singh@amd.com, michael.roth@amd.com,
        venu.busireddy@oracle.com, sterritt@google.com,
        tony.luck@intel.com, samitolvanen@google.com, fenghua.yu@intel.com,
        pangupta@amd.com, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org, linux-hyperv@vger.kernel.org,
        linux-arch@vger.kernel.org
Subject: Re: [RFC PATCH V6 01/14] x86/sev: Add a #HV exception handler
Message-ID: <20230530185232.GA211927@hirez.programming.kicks-ass.net>
References: <20230515165917.1306922-1-ltykernel@gmail.com>
 <20230515165917.1306922-2-ltykernel@gmail.com>
 <20230516093010.GC2587705@hirez.programming.kicks-ass.net>
 <d43c14d9-a149-860c-71d6-e5c62b7c356f@amd.com>
 <20230530143504.GA200197@hirez.programming.kicks-ass.net>
 <0f0ab135-cdd0-0691-e0c1-42645671fe15@amd.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <0f0ab135-cdd0-0691-e0c1-42645671fe15@amd.com>
Precedence: bulk

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.