From mboxrd@z Thu Jan  1 00:00:00 1970
From: Peter Zijlstra <peterz@infradead.org>
Subject: Re: Should SEV-ES #VC use IST? (Re: [PATCH] Allow RDTSC and RDTSCP
 from userspace)
Date: Tue, 23 Jun 2020 13:07:06 +0200
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To: Joerg Roedel <jroedel@suse.de>
Cc: Juergen Gross <JGross@suse.com>, Tom Lendacky <Thomas.Lendacky@amd.com>, Thomas Hellstrom <thellstrom@vmware.com>, X86 ML <x86@kernel.org>, Mike Stunes <mstunes@vmware.com>, Kees Cook <keescook@chromium.org>, kvm list <kvm@vger.kernel.org>, Andrew Cooper <andrew.cooper3@citrix.com>, Joerg Roedel <joro@8bytes.org>, Dave Hansen <dave.hansen@linux.intel.com>, LKML <linux-kernel@vger.kernel.org>, Sean Christopherson <sean.j.christopherson@intel.com>, Linux Virtualization <virtualization@lists.linux-foundation.org>, Dave Hansen <dave.hansen@intel.com>, Andy Lutomirski <luto@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>, Dan Williams <dan.j.williams@intel.com>, Jiri Slaby <jslaby@suse.cz>
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On Tue, Apr 28, 2020 at 09:55:12AM +0200, Joerg Roedel wrote:
> On Mon, Apr 27, 2020 at 10:37:41AM -0700, Andy Lutomirski wrote:
> > I have a somewhat serious question: should we use IST for #VC at all?
> > As I understand it, Rome and Naples make it mandatory for hypervisors
> > to intercept #DB, which means that, due to the MOV SS mess, it's sort
> > of mandatory to use IST for #VC.  But Milan fixes the #DB issue, so,
> > if we're running under a sufficiently sensible hypervisor, we don't
> > need IST for #VC.
> 
> The reason for #VC being IST is not only #DB, but also SEV-SNP. SNP adds
> page ownership tracking between guest and host, so that the hypervisor
> can't remap guest pages without the guest noticing.
> 
> If there is a violation of ownership, which can happen at any memory
> access, there will be a #VC exception to notify the guest. And as this
> can happen anywhere, for example on a carefully crafted stack page set
> by userspace before doing SYSCALL, the only robust choice for #VC is to
> use IST.

So what happens if this #VC triggers on the first access to the #VC
stack, because the malicious host has craftily mucked with only the #VC
IST stack page?

Or on the NMI IST stack, then we get #VC in NMI before the NMI can fix
you up.

AFAICT all of that is non-recoverable.