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Date:   Tue, 23 Jun 2020 14:47:12 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Andrew Cooper <andrew.cooper3@citrix.com>
Cc:     Joerg Roedel <jroedel@suse.de>, 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>
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 12:51:03PM +0100, Andrew Cooper wrote:

> There are cases which are definitely non-recoverable.
> 
> For both ES and SNP, a malicious hypervisor can mess with the guest
> physmap to make the the NMI, #VC and #DF stacks all alias.
> 
> For ES, this had better result in the #DF handler deciding that crashing
> is the way out, whereas for SNP, this had better escalate to Shutdown.

> Crashing out hard if the hypervisor is misbehaving is acceptable.

Then I'm thinking the only sensible option is to crash hard for any SNP
#VC from kernel mode.

Sadly that doesn't help with #VC needing to be IST :-( IST is such a
frigging nightmare.