Messages in this thread |  | | Date | Mon, 8 Jan 2018 14:26:11 -0000 | Subject | Re: [PATCH v6 01/10] x86/retpoline: Add initial retpoline support | From | "David Woodhouse" <> |
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> 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.
-- dwmw2
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