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Date:   Tue, 23 Jun 2020 17:38:44 +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 05:23:26PM +0200, Peter Zijlstra wrote:
> On Tue, Jun 23, 2020 at 04:59:14PM +0200, Joerg Roedel wrote:
> > On Tue, Jun 23, 2020 at 04:53:44PM +0200, Peter Zijlstra wrote:
> > > +noinstr void idtentry_validate_ist(struct pt_regs *regs)
> > > +{
> > > +	if ((regs->sp & ~(EXCEPTION_STKSZ-1)) ==
> > > +	    (_RET_IP_ & ~(EXCEPTION_STKSZ-1)))
> > > +		die("IST stack recursion", regs, 0);
> > > +}
> > 
> > Yes, this is a start, it doesn't cover the case where the NMI stack is
> > in-between, so I think you need to walk down regs->sp too.
> 
> That shouldn't be possible with the current code, I think.

To clarify, we have: NMI, MCE, DB and DF.

DF (with the exception of ESPFIX) is fatal.
MCE from kernel is fatal (which is what makes the MCE in NMI 'work')
NMI and DB clear DR7, which avoids DB in NMI.

So that leaves: NMI in DB, and that works.