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Message-ID: <CAKv+Gu9HpKBO-r+Ker47sPxvHBWLa6NAHe4P71x=K4Wiy2ybwQ@mail.gmail.com>
Date: Wed, 19 Feb 2020 08:40:47 +0100
From: Ard Biesheuvel <ard.biesheuvel@...aro.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>,
Mark Rutland <mark.rutland@....com>, James Morse <james.morse@....com>,
Dave Martin <Dave.Martin@....com>, Kees Cook <keescook@...omium.org>,
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 <clang-built-linux@...glegroups.com>,
Kernel Hardening <kernel-hardening@...ts.openwall.com>,
linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v8 12/12] efi/libstub: disable SCS
On Wed, 19 Feb 2020 at 01:09, Sami Tolvanen <samitolvanen@...gle.com> 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>
> ---
> 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
>
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