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

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Date:   Mon, 27 Apr 2020 10:37:41 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Joerg Roedel <joro@...tes.org>, Joerg Roedel <jroedel@...e.de>,
        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>,
        Peter Zijlstra <peterz@...radead.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: Should SEV-ES #VC use IST? (Re: [PATCH] Allow RDTSC and RDTSCP from userspace)

On Sat, Apr 25, 2020 at 3:10 PM Andy Lutomirski <luto@...nel.org> wrote:
>
> On Sat, Apr 25, 2020 at 1:23 PM Joerg Roedel <joro@...tes.org> wrote:
> >
> > On Sat, Apr 25, 2020 at 12:47:31PM -0700, Andy Lutomirski wrote:
> > > I assume the race you mean is:
> > >
> > > #VC
> > > Immediate NMI before IST gets shifted
> > > #VC
> > >
> > > Kaboom.
> > >
> > > How are you dealing with this?  Ultimately, I think that NMI will need
> > > to turn off IST before engaging in any funny business. Let me ponder
> > > this a bit.
> >
> > Right, I dealt with that by unconditionally shifting/unshifting the #VC IST entry
> > in do_nmi() (thanks to Davin Kaplan for the idea). It might cause
> > one of the IST stacks to be unused during nesting, but that is fine. The
> > stack memory for #VC is only allocated when SEV-ES is active (in an
> > SEV-ES VM).
>
> Blech.  It probably works, but still, yuck.  It's a bit sad that we
> seem to be growing more and more poorly designed happens-anywhere
> exception types at an alarming rate.  We seem to have #NM, #MC, #VC,
> #HV, and #DB.  This doesn't really scale.

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.

So I think we have two choices:

1. Use IST for #VC and deal with all the mess that entails.

2. Say that we SEV-ES client support on Rome and Naples is for
development only and do a quick boot-time check for whether #DB is
intercepted.  (Just set TF and see what vector we get.)  If #DB is
intercepted, print a very loud warning and refuse to boot unless some
special sev_es.insecure_development_mode or similar option is set.

#2 results in simpler and more robust entry code.  #1 is more secure.

So my question is: will anyone actually use SEV-ES in production on
Rome or Naples?  As I understand it, it's not really ready for prime
time on those chips.  And do we care if the combination of a malicious
hypervisor and malicious guest userspace on Milan can compromise the
guest kernel?  I don't think SEV-ES is really mean to resist a
concerted effort by the hypervisor to compromise the guest.

--Andy