Re: [PATCH v3 12/17] arm64: reserve x18 from general allocation with SCS

Kees Cook
2019-11-01T04:00:19Z

On Thu, Oct 31, 2019 at 09:46:32AM -0700, samitolvanen@google.com wrote:
> Reserve the x18 register from general allocation when SCS is enabled,
> because the compiler uses the register to store the current task's
> shadow stack pointer. Note that all external kernel modules must also be
> compiled with -ffixed-x18 if the kernel has SCS enabled.
>
> Signed-off-by: Sami Tolvanen <samitolvanen@google.com>

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

-Kees

> ---
> arch/arm64/Makefile | 4 ++++
> 1 file changed, 4 insertions(+)
>
> diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
> index 2c0238ce0551..ef76101201b2 100644
> --- a/arch/arm64/Makefile
> +++ b/arch/arm64/Makefile
> @@ -72,6 +72,10 @@ stack_protector_prepare: prepare0
> include/generated/asm-offsets.h))
> endif
>
> +ifeq ($(CONFIG_SHADOW_CALL_STACK), y)
> +KBUILD_CFLAGS += -ffixed-x18
> +endif
> +
> ifeq ($(CONFIG_CPU_BIG_ENDIAN), y)
> KBUILD_CPPFLAGS += -mbig-endian
> CHECKFLAGS += -D__AARCH64EB__
> --
> 2.24.0.rc0.303.g954a862665-goog
>

--
Kees Cook