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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Fri, 18 Oct 2019 14:23:10 -0700
Message-ID: <CAKwvOd=rU2cC7C3a=8D2WBEmS49YgR7=aCriE31JQx7ExfQZrg@mail.gmail.com>
Subject: Re: [PATCH 12/18] arm64: reserve x18 only with Shadow Call Stack
To:     Sami Tolvanen <samitolvanen@google.com>
Cc:     Will Deacon <will@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Dave Martin <Dave.Martin@arm.com>,
        Kees Cook <keescook@chromium.org>,
        Laura Abbott <labbott@redhat.com>,
        Mark Rutland <mark.rutland@arm.com>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Kernel Hardening <kernel-hardening@lists.openwall.com>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        LKML <linux-kernel@vger.kernel.org>
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Sender: linux-kernel-owner@vger.kernel.org
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On Fri, Oct 18, 2019 at 9:11 AM 'Sami Tolvanen' via Clang Built Linux
<clang-built-linux@googlegroups.com> wrote:
>
> Only reserve x18 with CONFIG_SHADOW_CALL_STACK. Note that all external
> kernel modules must also have x18 reserved if the kernel uses SCS.

Ah, ok.  The tradeoff for maintainers to consider, either:
1. one less GPR for ALL kernel code or
2. remember not to use x18 in inline as lest you potentially break SCS

This patch is 2 (the earlier patch was 1).  Maybe we don't write
enough inline asm that this will be hard to remember, and we do have
CI in Android to watch for this (on mainline, not sure about -next).

Either way,
Acked-by: Nick Desaulniers <ndesaulniers@google.com>

>
> Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
> ---
>  arch/arm64/Makefile | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
> index 1c7b276bc7c5..ef76101201b2 100644
> --- a/arch/arm64/Makefile
> +++ b/arch/arm64/Makefile
> @@ -55,7 +55,7 @@ endif
>
>  KBUILD_CFLAGS  += -mgeneral-regs-only $(lseinstr) $(brokengasinst)     \
>                    $(compat_vdso) $(cc_has_k_constraint)
> -KBUILD_CFLAGS  += -fno-asynchronous-unwind-tables -ffixed-x18
> +KBUILD_CFLAGS  += -fno-asynchronous-unwind-tables
>  KBUILD_CFLAGS  += $(call cc-disable-warning, psabi)
>  KBUILD_AFLAGS  += $(lseinstr) $(brokengasinst) $(compat_vdso)
>
> @@ -72,6 +72,10 @@ stack_protector_prepare: prepare0
>                                         include/generated/asm-offsets.h))
>  endif
>
> +ifeq ($(CONFIG_SHADOW_CALL_STACK), y)
> +KBUILD_CFLAGS  += -ffixed-x18
> +endif
> +
>  ifeq ($(CONFIG_CPU_BIG_ENDIAN), y)
>  KBUILD_CPPFLAGS        += -mbig-endian
>  CHECKFLAGS     += -D__AARCH64EB__
> --
> 2.23.0.866.gb869b98d4c-goog
>
> --
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-- 
Thanks,
~Nick Desaulniers