linux-kernel - Re: [PATCH 5/5] x86/vsyscall: Change the default vsyscall mode to xonly

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Message-ID: <CALCETrUYNavL8pu4jQqJjoT=PdeRyjeoLDn=0r7h=2XsHDMezQ@mail.gmail.com>
Date:   Thu, 13 Jun 2019 12:14:50 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Borislav Petkov <bp@...en8.de>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>
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 <keescook@...omium.org> 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?

--Andy