[PATCH 4.9 38/43] x86/build: Disable CET instrumentation in the kernel

Greg Kroah-Hartman
2021-02-08T15:48:15Z

From: Josh Poimboeuf <jpoimboe@redhat.com>

commit 20bf2b378729c4a0366a53e2018a0b70ace94bcd upstream.

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
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
Makefile | 6 ------
arch/x86/Makefile | 3 +++
2 files changed, 3 insertions(+), 6 deletions(-)

--- a/Makefile
+++ b/Makefile
@@ -841,12 +841,6 @@ KBUILD_CFLAGS += $(call cc-option,-Wer
# change __FILE__ to the relative path from the srctree
KBUILD_CFLAGS += $(call cc-option,-fmacro-prefix-map=$(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
-
# use the deterministic mode of AR if available
KBUILD_ARFLAGS := $(call ar-option,D)

--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -137,6 +137,9 @@ 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)
+
# -funit-at-a-time shrinks the kernel .text considerably
# unfortunately it makes reading oopses harder.
KBUILD_CFLAGS += $(call cc-option,-funit-at-a-time)