x86/build: Disable CET instrumentation in the kernel

With retpolines disabled, some configurations of GCC, and specifically
the GCC versions 9 and 10 in Ubuntu will add Intel CET instrumentation
to the kernel by default. That breaks certain tracing scenarios by
adding a superfluous ENDBR64 instruction before the fentry call, for
functions which can be called indirectly.

CET instrumentation isn't currently necessary in the kernel, as CET is
only supported in user space. Disable it unconditionally and move it
into the x86's Makefile as CET/CFI... enablement should be a per-arch
decision anyway.

 [ bp: Massage and extend commit message. ]

Fixes: 29be86d7f9 ("kbuild: add -fcf-protection=none when using retpoline flags")
Reported-by: Nikolay Borisov <[email protected]>
Signed-off-by: Josh Poimboeuf <[email protected]>
Signed-off-by: Borislav Petkov <[email protected]>
Reviewed-by: Nikolay Borisov <[email protected]>
Tested-by: Nikolay Borisov <[email protected]>
Cc: <[email protected]>
Cc: Seth Forshee <[email protected]>
Cc: Masahiro Yamada <[email protected]>
Link: https://lkml.kernel.org/r/20210128215219.6kct3h2eiustncws@treble

Makefile

@@ -948,12 +948,6 @@ KBUILD_CFLAGS   += $(call cc-option,-Werror=designated-init)
 # change __FILE__ to the relative path from the srctree
 KBUILD_CPPFLAGS += $(call cc-option,-fmacro-prefix-map=$(srctree)/=)
 
-# ensure -fcf-protection is disabled when using retpoline as it is
-# incompatible with -mindirect-branch=thunk-extern
-ifdef CONFIG_RETPOLINE
-KBUILD_CFLAGS += $(call cc-option,-fcf-protection=none)
-endif
-
 # include additional Makefiles when needed
 include-y			:= scripts/Makefile.extrawarn
 include-$(CONFIG_KASAN)		+= scripts/Makefile.kasan

arch/x86/Makefile

@@ -120,6 +120,9 @@ else
 
         KBUILD_CFLAGS += -mno-red-zone
         KBUILD_CFLAGS += -mcmodel=kernel
+
+	# Intel CET isn't enabled in the kernel
+	KBUILD_CFLAGS += $(call cc-option,-fcf-protection=none)
 endif
 
 ifdef CONFIG_X86_X32