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Date:   Tue, 23 Jun 2020 15:59:16 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Joerg Roedel <jroedel@suse.de>
Cc:     Andy Lutomirski <luto@kernel.org>, Joerg Roedel <joro@8bytes.org>,
        Dave Hansen <dave.hansen@intel.com>,
        Tom Lendacky <Thomas.Lendacky@amd.com>,
        Mike Stunes <mstunes@vmware.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Juergen Gross <JGross@suse.com>,
        Jiri Slaby <jslaby@suse.cz>, Kees Cook <keescook@chromium.org>,
        kvm list <kvm@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Thomas Hellstrom <thellstrom@vmware.com>,
        Linux Virtualization <virtualization@lists.linux-foundation.org>,
        X86 ML <x86@kernel.org>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        Andrew Cooper <andrew.cooper3@citrix.com>
Subject: Re: Should SEV-ES #VC use IST? (Re: [PATCH] Allow RDTSC and RDTSCP
 from userspace)
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On Tue, Jun 23, 2020 at 03:40:03PM +0200, Joerg Roedel wrote:
> On Tue, Jun 23, 2020 at 02:52:01PM +0200, Peter Zijlstra wrote:

> > You only have that guarantee when any SNP #VC from kernel is an
> > automatic panic. But in that case, what's the point of having the
> > recursion count?
> 
> It is not a recursion count, it is a stack-recursion check. Basically
> walk down the stack and look if your current stack is already in use.
> Yes, this can be optimized, but that is what is needed.
> 
> IIRC the current prototype code for SNP just pre-validates all memory in
> the VM and doesn't support moving pages around on the host. So any #VC
> SNP exception would be fatal, yes.
> 
> In a scenario with on-demand validation of guest pages and support for
> guest-assisted page-moving on the HV side it would be more complicated.
> Basically all memory that is accessed during #VC exception handling must
> stay validated at all times, including the IST stack.
> 
> So saying this, I don't understand why _all_ SNP #VC exceptions from
> kernel space must be fatal?

Ah, because I hadn't thought of the stack-recursion check.

So basically when your exception frame points to your own IST, you die.
That sounds like something we should have in generic IST code.