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Date:   Tue, 23 Jun 2020 17:23:26 +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 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.

> The dumpstack code already has some logic for this.

Reliability of that depends on the unwinder, I wouldn't want the guess
uwinder to OOPS me by accident.