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

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Date:   Tue, 23 Jun 2020 14:52:01 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Joerg Roedel <jroedel@...e.de>
Cc:     Andy Lutomirski <luto@...nel.org>, Joerg Roedel <joro@...tes.org>,
        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>,
        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: Re: Should SEV-ES #VC use IST? (Re: [PATCH] Allow RDTSC and RDTSCP
 from userspace)

On Tue, Jun 23, 2020 at 02:04:33PM +0200, Joerg Roedel wrote:
> On Tue, Jun 23, 2020 at 01:48:18PM +0200, Peter Zijlstra wrote:
> > On Tue, Jun 23, 2020 at 01:30:07PM +0200, Joerg Roedel wrote:
> 
> > But you cannot do a recursion check in #VC, because the NMI can happen
> > on the first instruction of #VC, before we can increment our counter,
> > and then the #VC can happen on NMI because the IST stack is a goner, and
> > we're fscked again (or on a per-cpu variable we touch in our elaborate
> > NMI setup, etc..).
> 
> No, the recursion check is fine, because overwriting an already used IST
> stack doesn't matter (as long as it can be detected) if we are going to
> panic anyway. It doesn't matter because the kernel will not leave the
> currently running handler anymore.

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?

> > I'll keep repeating this, x86_64 exceptions are a trainwreck, and IST in
> > specific is utter crap.
> 
> I agree, but don't forget the most prominent underlying reason for IST:
> The SYSCALL gap. If SYSCALL would switch stacks most of those issues
> would not exist. IST would still be needed because there are no task
> gates in x86-64, but still...

We could all go back to int80 ;-) /me runs like heck