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Date: Thu, 13 Jun 2019 22:19:23 -0700
From: Kees Cook <>
To: Andy Lutomirski <>
Cc: X86 ML <>, LKML <>,
	Borislav Petkov <>,
	Kernel Hardening <>,
	Peter Zijlstra <>,
	Thomas Gleixner <>
Subject: Re: [PATCH 5/5] x86/vsyscall: Change the default vsyscall mode to

On Thu, Jun 13, 2019 at 12:14:50PM -0700, Andy Lutomirski wrote:
> On Mon, Jun 10, 2019 at 1:44 PM Kees Cook <> wrote:
> >
> > On Mon, Jun 10, 2019 at 01:25:31PM -0700, Andy Lutomirski wrote:
> > > The use case for full emulation over xonly is very esoteric.  Let's
> > > change the default to the safer xonly mode.
> >
> > Perhaps describe the esoteric cases here (and maybe in the Kconfig help
> > text)? That should a user determine if they actually need it. (What
> > would the failure under xonly look like for someone needing emulate?)
> I added it to the Kconfig text.
> Right now, the failure will just be a segfault.  I could add some
> logic so that it would log "invalid read to vsyscall page -- fix your
> userspace or boot with vsyscall=emulate".  Do you think that's
> important?

I think it would be a friendly way to help anyone wondering why
something suddenly started segfaulting, yeah. Just a pr_warn_once() or
something (not a WARN() since it's "intentionally" reachable by

Kees Cook

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