Re: [PATCH v6 01/10] x86/retpoline: Add initial retpoline support

[Josh Poimboeuf <jpoimboe@redhat.com>]

On Mon, Jan 08, 2018 at 02:26:11PM -0000, David Woodhouse wrote:
> 
> > On Mon, Jan 08, 2018 at 02:46:32PM +0100, Thomas Gleixner wrote:
> >> On Mon, 8 Jan 2018, Josh Poimboeuf wrote:
> >> > On Sun, Jan 07, 2018 at 10:11:16PM +0000, David Woodhouse wrote:
> >> > > diff --git a/arch/x86/Makefile b/arch/x86/Makefile
> >> > > index a20eacd..918e550 100644
> >> > > --- a/arch/x86/Makefile
> >> > > +++ b/arch/x86/Makefile
> >> > > @@ -235,6 +235,16 @@ KBUILD_CFLAGS += -Wno-sign-compare
> >> > >  #
> >> > >  KBUILD_CFLAGS += -fno-asynchronous-unwind-tables
> >> > >
> >> > > +# Avoid indirect branches in kernel to deal with Spectre
> >> > > +ifdef CONFIG_RETPOLINE
> >> > > +    RETPOLINE_CFLAGS += $(call
> >> cc-option,-mindirect-branch=thunk-extern -mindirect-branch-register)
> >> > > +    ifneq ($(RETPOLINE_CFLAGS),)
> >> > > +        KBUILD_CFLAGS += $(RETPOLINE_CFLAGS) -DRETPOLINE
> >> > > +    else
> >> > > +        $(warning Retpoline not supported in compiler. System may
> >> be insecure.)
> >> > > +    endif
> >> > > +endif
> >> >
> >> > I wonder if an error might be more appropriate than a warning.  I
> >> > learned from experience that a lot of people don't see these Makefile
> >> > warnings, and this would be a dangerous one to miss.
> >> >
> >> > Also if this were an error, you could get rid of the RETPOLINE define,
> >> > and that would be one less define cluttering up the already
> >> way-too-long
> >> > GCC arg list.
> >>
> >> It still allows to get the ASM part covered. If that's worth it I can't
> >> tell.
> >
> > If there's a makefile error above, then CONFIG_RETPOLINE would already
> > imply compiler support, so the ASM code with the new '%V' option could
> > just do 'ifdef CONFIG_RETPOLINE'.
> 
> I did look at ditching the -DRETPOLINE but there is benefit in doing the
> sys_call_table jump even when GCC isn't updated. So I put it back.

What benefit is that?  Doesn't it give the user a false sense of
security, since there's no shortage of other indirect branches to
attack?

-- 
Josh