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Date:   Mon, 7 Mar 2022 12:17:19 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     x86@kernel.org, joao@overdrivepizza.com, hjl.tools@gmail.com,
        jpoimboe@redhat.com, andrew.cooper3@citrix.com
Cc:     linux-kernel@vger.kernel.org, ndesaulniers@google.com,
        keescook@chromium.org, samitolvanen@google.com,
        mark.rutland@arm.com, alyssa.milburn@intel.com, mbenes@suse.cz,
        rostedt@goodmis.org, mhiramat@kernel.org,
        alexei.starovoitov@gmail.com
Subject: Re: [PATCH v3 04/39] x86/ibt: Base IBT bits
Message-ID: <YiXpv0q88paPHPqF@hirez.programming.kicks-ass.net>
References: <20220303112321.422525803@infradead.org>
 <20220303112825.190662703@infradead.org>
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On Thu, Mar 03, 2022 at 12:23:25PM +0100, Peter Zijlstra wrote:

> --- a/arch/x86/Makefile
> +++ b/arch/x86/Makefile
> @@ -62,8 +62,11 @@ export BITS
>  #
>  KBUILD_CFLAGS += -mno-sse -mno-mmx -mno-sse2 -mno-3dnow -mno-avx
>  
> -# Intel CET isn't enabled in the kernel
> +ifeq ($(CONFIG_X86_KERNEL_IBT),y)
> +KBUILD_CFLAGS += $(call cc-option,-fcf-protection=branch)
> +else
>  KBUILD_CFLAGS += $(call cc-option,-fcf-protection=none)
> +endif
>  
>  ifeq ($(CONFIG_X86_32),y)
>          BITS := 32

Joao reported that RETPOLINE=n builds explode; turns out the compilers
default to using NOTRACK prefixes for jump-tables and we explicitly do
not enable that security compromise for the kernel.

Since the compilers don't have explicit control over NOTRACK generation,
blanket disable jump-tables when using IBT without RETPOLINE.

Joao will be submitting GCC and Clang bugreports on this shortly.

diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index d38c18f4bd53..f80a425e7d29 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -63,7 +63,9 @@ export BITS
 KBUILD_CFLAGS += -mno-sse -mno-mmx -mno-sse2 -mno-3dnow -mno-avx
 
 ifeq ($(CONFIG_X86_KERNEL_IBT),y)
-KBUILD_CFLAGS += $(call cc-option,-fcf-protection=branch)
+# Explicitly disable jump-tables, also implied by RETPOLINE=y, for kernel IBT
+# to avoid NOTRACK prefixes.
+KBUILD_CFLAGS += $(call cc-option,-fcf-protection=branch -fno-jump-tables)
 else
 KBUILD_CFLAGS += $(call cc-option,-fcf-protection=none)
 endif