Re: [PATCH] kbuild: Provide way to actually disable stack protector

Masahiro Yamada
2020-06-23T02:34:56Z

On Tue, Jun 23, 2020 at 4:02 AM Kees Cook <keescook@chromium.org> wrote:
>
> Some builds of GCC enable stack protector by default. Simply removing
> the arguments is not sufficient to disable stack protector, as the stack
> protector for those GCC builds must be explicitly disabled. (Removing the
> arguments is left as-is just to be sure there are no ordering problems. If
> -fno-stack-protector ended up _before_ -fstack-protector, it would not
> disable it: GCC uses whichever -f... comes last on the command line.)
>
> Fixes: 20355e5f73a7 ("x86/entry: Exclude low level entry code from sanitizing")
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
> Makefile | 4 +++-
> arch/Kconfig | 3 ---
> arch/arm/boot/compressed/Makefile | 4 ++--
> arch/x86/entry/Makefile | 3 +++
> 4 files changed, 8 insertions(+), 6 deletions(-)
>
> diff --git a/Makefile b/Makefile
> index ac2c61c37a73..b46e91bf0b0e 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -762,7 +762,9 @@ ifneq ($(CONFIG_FRAME_WARN),0)
> KBUILD_CFLAGS += -Wframe-larger-than=$(CONFIG_FRAME_WARN)
> endif
>
> -stackp-flags-$(CONFIG_CC_HAS_STACKPROTECTOR_NONE) := -fno-stack-protector
> +DISABLE_STACKPROTECTOR := $(call cc-option,-fno-stack-protector)
> +export DISABLE_STACKPROTECTOR
> +stackp-flags-y := $(DISABLE_STACKPROTECTOR)
> stackp-flags-$(CONFIG_STACKPROTECTOR) := -fstack-protector
> stackp-flags-$(CONFIG_STACKPROTECTOR_STRONG) := -fstack-protector-strong
>
> diff --git a/arch/Kconfig b/arch/Kconfig
> index 8cc35dc556c7..1ea61290900a 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -478,9 +478,6 @@ config HAVE_STACKPROTECTOR
> An arch should select this symbol if:
> - it has implemented a stack canary (e.g. __stack_chk_guard)
>
> -config CC_HAS_STACKPROTECTOR_NONE
> - def_bool $(cc-option,-fno-stack-protector)
> -
> config STACKPROTECTOR
> bool "Stack Protector buffer overflow detection"
> depends on HAVE_STACKPROTECTOR
> diff --git a/arch/arm/boot/compressed/Makefile b/arch/arm/boot/compressed/Makefile
> index 00602a6fba04..3693bac525d2 100644
> --- a/arch/arm/boot/compressed/Makefile
> +++ b/arch/arm/boot/compressed/Makefile
> @@ -84,9 +84,9 @@ endif
>
> # -fstack-protector-strong triggers protection checks in this code,
> # but it is being used too early to link to meaningful stack_chk logic.
> -nossp-flags-$(CONFIG_CC_HAS_STACKPROTECTOR_NONE) := -fno-stack-protector
> $(foreach o, $(libfdt_objs) atags_to_fdt.o, \
> - $(eval CFLAGS_$(o) := -I $(srctree)/scripts/dtc/libfdt $(nossp-flags-y)))
> + $(eval CFLAGS_$(o) := -I $(srctree)/scripts/dtc/libfdt \
> + $(DISABLE_STACKPROTECTOR)))
>
> # These were previously generated C files. When you are building the kernel
> # with O=, make sure to remove the stale files in the output tree. Otherwise,
> diff --git a/arch/x86/entry/Makefile b/arch/x86/entry/Makefile
> index b7a5790d8d63..79902decc3d1 100644
> --- a/arch/x86/entry/Makefile
> +++ b/arch/x86/entry/Makefile
> @@ -10,6 +10,9 @@ KCOV_INSTRUMENT := n
> CFLAGS_REMOVE_common.o = $(CC_FLAGS_FTRACE) -fstack-protector -fstack-protector-strong
> CFLAGS_REMOVE_syscall_32.o = $(CC_FLAGS_FTRACE) -fstack-protector -fstack-protector-strong
> CFLAGS_REMOVE_syscall_64.o = $(CC_FLAGS_FTRACE) -fstack-protector -fstack-protector-strong
> +CFLAGS_common.o += $(DISABLE_STACKPROTECTOR)
> +CFLAGS_syscall_32.o += $(DISABLE_STACKPROTECTOR)
> +CFLAGS_syscall_64.o += $(DISABLE_STACKPROTECTOR)

There is one more c file in this directory.

Is it OK to not patch syscall_x32.c ?

>
> CFLAGS_syscall_64.o += $(call cc-option,-Wno-override-init,)
> CFLAGS_syscall_32.o += $(call cc-option,-Wno-override-init,)

This patch is ugly.

I'd rather want to fix this by one-liner.

diff --git a/arch/x86/entry/Makefile b/arch/x86/entry/Makefile
index b7a5790d8d63..0d41eb91aaea 100644
--- a/arch/x86/entry/Makefile
+++ b/arch/x86/entry/Makefile
@@ -11,6 +11,8 @@ CFLAGS_REMOVE_common.o = $(CC_FLAGS_FTRACE)
-fstack-protector -fstack-protector-
CFLAGS_REMOVE_syscall_32.o = $(CC_FLAGS_FTRACE) -fstack-protector
-fstack-protector-strong
CFLAGS_REMOVE_syscall_64.o = $(CC_FLAGS_FTRACE) -fstack-protector
-fstack-protector-strong

+ccflags-$(CONFIG_CC_HAS_STACKPROTECTOR_NONE) += -fno-stack-protector
+
CFLAGS_syscall_64.o += $(call cc-option,-Wno-override-init,)
CFLAGS_syscall_32.o += $(call cc-option,-Wno-override-init,)
obj-y := entry_$(BITS).o thunk_$(BITS).o
syscall_$(BITS).o

--
Best Regards
Masahiro Yamada