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From:   Ard Biesheuvel <ard.biesheuvel@linaro.org>
Date:   Wed, 19 Feb 2020 08:40:47 +0100
Message-ID: <CAKv+Gu9HpKBO-r+Ker47sPxvHBWLa6NAHe4P71x=K4Wiy2ybwQ@mail.gmail.com>
Subject: Re: [PATCH v8 12/12] efi/libstub: disable SCS
To:     Sami Tolvanen <samitolvanen@google.com>
Cc:     Will Deacon <will@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        James Morse <james.morse@arm.com>,
        Dave Martin <Dave.Martin@arm.com>,
        Kees Cook <keescook@chromium.org>,
        Laura Abbott <labbott@redhat.com>,
        Marc Zyngier <maz@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Jann Horn <jannh@google.com>,
        Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>,
        Masahiro Yamada <yamada.masahiro@socionext.com>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Kernel Hardening <kernel-hardening@lists.openwall.com>,
        linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Wed, 19 Feb 2020 at 01:09, Sami Tolvanen <samitolvanen@google.com> wrote:
>
> Disable SCS for the EFI stub and allow x18 to be used.
>
> Suggested-by: James Morse <james.morse@arm.com>
> Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
> ---
>  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))
> +

I don't see why you'd need to remove -ffixed-x18 again here. Not using
x18 anywhere in the kernel is a much more maintainable approach.

In fact, now that I think of it, the EFI AArch64 platform binding
forbids the use of x18, so it would be better to add the -ffixed-x18
unconditionally for arm64 (even though the reason it forbids it is to
ensure compatibility with an OS using it as a platform register, and
so nothing is actually broken atm).

>  GCOV_PROFILE                   := n
>  KASAN_SANITIZE                 := n
>  UBSAN_SANITIZE                 := n
> --
> 2.25.0.265.gbab2e86ba0-goog
>