From 20bf2b378729c4a0366a53e2018a0b70ace94bcd Mon Sep 17 00:00:00 2001
From: Josh Poimboeuf <jpoimboe@redhat.com>
Date: Thu, 28 Jan 2021 15:52:19 -0600
Subject: [PATCH] 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: 29be86d7f9cb ("kbuild: add -fcf-protection=none when using retpoline flags")
Reported-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Tested-by: Nikolay Borisov <nborisov@suse.com>
Cc: <stable@vger.kernel.org>
Cc: Seth Forshee <seth.forshee@canonical.com>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Link: https://lkml.kernel.org/r/20210128215219.6kct3h2eiustncws@treble
---
Makefile | 6 ------
arch/x86/Makefile | 3 +++
2 files changed, 3 insertions(+), 6 deletions(-)
diff --git a/Makefile b/Makefile
index e0af7a4a55985..51c2bf34142de 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 7116da3980be4..5857917f83eee 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
--
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