x86/build: Disable CET instrumentation in the kernel

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>

diff --git a/Makefile b/Makefile
index e4c6b6e62edb98e701a8b1c380cd4e9854a79f41..9e126dc1bc404669ba126fbd7fd670a2a717cd92 100644 (file)
--- a/Makefile
+++ b/Makefile
@@ -830,12 +830,6 @@ KBUILD_CFLAGS   += $(call cc-option,-Werror=strict-prototypes)
 # Prohibit date/time macros, which would make the build non-deterministic
 KBUILD_CFLAGS   += $(call cc-option,-Werror=date-time)
 
-# 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)
 

diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index 8b4d022ce0cbc89c2ef4b8e1513b53ae0edc41ef..e59dc138b24ea67068d4088237f2ca7f5dbcd939 100644 (file)
--- 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)