Re: [PATCH 2/3] Makefile: Move stackprotector availability out of Kconfig

[Masahiro Yamada]

Hi Kees,


2017-10-03 4:20 GMT+09:00 Kees Cook <keescook@xxxxxxxxxxxx>:
> Various portions of the kernel, especially per-architecture pieces,
> need to know if the compiler is building it with the stack protector.
> This was done in the arch/Kconfig with 'select', but this doesn't
> allow a way to do auto-detected compiler support. In preparation for
> creating an on-if-available default, move the logic for the definition of
> CONFIG_CC_STACKPROTECTOR into the Makefile.
>
> Cc: Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx>
> Cc: Michal Marek <mmarek@xxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Laura Abbott <labbott@xxxxxxxxxx>
> Cc: Nicholas Piggin <npiggin@xxxxxxxxx>
> Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> Cc: linux-kbuild@xxxxxxxxxxxxxxx
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
>  Makefile     | 7 +++++--
>  arch/Kconfig | 8 --------
>  2 files changed, 5 insertions(+), 10 deletions(-)
>
> diff --git a/Makefile b/Makefile
> index d1119941261c..e122a9cf0399 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -688,8 +688,11 @@ else
>    stackp-flag := $(call cc-option, -fno-stack-protector)
>  endif
>  endif
> -# Find arch-specific stack protector compiler sanity-checking script.
> -ifdef CONFIG_CC_STACKPROTECTOR
> +ifdef stackp-name
> +  # If the stack protector has been selected, inform the rest of the build.
> +  KBUILD_CFLAGS += -DCONFIG_CC_STACKPROTECTOR
> +  KBUILD_AFLAGS += -DCONFIG_CC_STACKPROTECTOR
> +  # Find arch-specific stack protector compiler sanity-checking script.
>    stackp-path := $(srctree)/scripts/gcc-$(SRCARCH)_$(BITS)-has-stack-protector.sh
>    stackp-check := $(wildcard $(stackp-path))
>  endif


I have not tested this series,
but I think this commit is bad (with the follow-up patch applied).


I thought of this scenario:

[1] Kernel is configured with CONFIG_CC_STACKPROTECTOR_AUTO

[2] Kernel is built with a compiler without stack protector support.

[3] CONFIG_CC_STACKPROTECTOR is not defined,
    so __stack_chk_fail() is not compiled.

[4] Out-of-tree modules are compiled with a compiler with
    stack protector support.
    __stack_chk_fail() is inserted to functions of the modules.

[5] insmod fails because reference to __stack_chk_fail()
    can not be resolved.




I think "select CC_STACKPROTECTOR" should be kept in Kconfig.




-- 
Best Regards
Masahiro Yamada