linux-kernel - [PATCH] x86: Disable CET instrumentation in the kernel

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Message-ID: <20210128215219.6kct3h2eiustncws@treble>
Date:   Thu, 28 Jan 2021 15:52:19 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     x86@...nel.org
Cc:     Masami Hiramatsu <masami.hiramatsu@...il.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>, bpf@...r.kernel.org,
        Steven Rostedt <rostedt@...dmis.org>,
        Nikolay Borisov <nborisov@...e.com>
Subject: [PATCH] x86: Disable CET instrumentation in the kernel


With retpolines disabled, some configurations of GCC 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.

Reported-by: Nikolay Borisov <nborisov@...e.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>
---
 Makefile          | 6 ------
 arch/x86/Makefile | 3 +++
 2 files changed, 3 insertions(+), 6 deletions(-)

diff --git a/Makefile b/Makefile
index e0af7a4a5598..51c2bf34142d 100644
--- a/Makefile
+++ b/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
diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index 32dcdddc1089..109c7f86483c 100644
--- a/arch/x86/Makefile
+++ b/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
-- 
2.29.2