linux-kernel - Re: [PATCH v8 12/12] efi/libstub: disable SCS

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Message-ID: <202002181658.45F66E21F@keescook>
Date:   Tue, 18 Feb 2020 16:58:56 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Mark Rutland <mark.rutland@....com>, james.morse@....com,
        Dave Martin <Dave.Martin@....com>,
        Laura Abbott <labbott@...hat.com>,
        Marc Zyngier <maz@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Jann Horn <jannh@...gle.com>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        clang-built-linux@...glegroups.com,
        kernel-hardening@...ts.openwall.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v8 12/12] efi/libstub: disable SCS

On Tue, Feb 18, 2020 at 04:08:17PM -0800, Sami Tolvanen wrote:
> Disable SCS for the EFI stub and allow x18 to be used.
> 
> Suggested-by: James Morse <james.morse@....com>
> Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>

Reviewed-by: Kees Cook <keescook@...omium.org>

-Kees

> ---
>  drivers/firmware/efi/libstub/Makefile | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/firmware/efi/libstub/Makefile b/drivers/firmware/efi/libstub/Makefile
> index 98a81576213d..dff9fa5a3f1c 100644
> --- a/drivers/firmware/efi/libstub/Makefile
> +++ b/drivers/firmware/efi/libstub/Makefile
> @@ -30,6 +30,9 @@ KBUILD_CFLAGS			:= $(cflags-y) -DDISABLE_BRANCH_PROFILING \
>  				   $(call cc-option,-fno-stack-protector) \
>  				   -D__DISABLE_EXPORTS
>  
> +#  remove SCS flags from all objects in this directory
> +KBUILD_CFLAGS := $(filter-out -ffixed-x18 $(CC_FLAGS_SCS), $(KBUILD_CFLAGS))
> +
>  GCOV_PROFILE			:= n
>  KASAN_SANITIZE			:= n
>  UBSAN_SANITIZE			:= n
> -- 
> 2.25.0.265.gbab2e86ba0-goog
> 

-- 
Kees Cook