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

[Kees Cook]

On Thu, Jun 13, 2019 at 12:14:50PM -0700, Andy Lutomirski wrote:
> On Mon, Jun 10, 2019 at 1:44 PM Kees Cook <keescook@xxxxxxxxxxxx> 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
userspace).

-- 
Kees Cook