linux-kernel - Re: Should SEV-ES #VC use IST? (Re: [PATCH] Allow RDTSC and RDTSCP from userspace)

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Message-ID: <20200623110706.GB4817@hirez.programming.kicks-ass.net>
Date:   Tue, 23 Jun 2020 13:07:06 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Joerg Roedel <jroedel@...e.de>
Cc:     Andy Lutomirski <luto@...nel.org>, Joerg Roedel <joro@...tes.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Tom Lendacky <Thomas.Lendacky@....com>,
        Mike Stunes <mstunes@...are.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>, Juergen Gross <JGross@...e.com>,
        Jiri Slaby <jslaby@...e.cz>, Kees Cook <keescook@...omium.org>,
        kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Thomas Hellstrom <thellstrom@...are.com>,
        Linux Virtualization <virtualization@...ts.linux-foundation.org>,
        X86 ML <x86@...nel.org>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Andrew Cooper <andrew.cooper3@...rix.com>
Subject: Re: Should SEV-ES #VC use IST? (Re: [PATCH] Allow RDTSC and RDTSCP
 from userspace)

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.