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Date: Thu, 13 Jun 2019 12:14:50 -0700
From: Andy Lutomirski <>
To: Kees Cook <>
Cc: Andy Lutomirski <>, X86 ML <>, LKML <>, 
	Borislav Petkov <>, Kernel Hardening <>, 
	Peter Zijlstra <>, Thomas Gleixner <>
Subject: Re: [PATCH 5/5] x86/vsyscall: Change the default vsyscall mode to xonly

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


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