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

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Message-ID: <CAKwvOdmBcHuhZZV54-76zY60F0Tvd_0DGCi+5dzFTxk0upOYrQ@mail.gmail.com>
Date:   Thu, 31 Oct 2019 10:11:16 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
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>,
        Dave Martin <Dave.Martin@....com>,
        Kees Cook <keescook@...omium.org>,
        Laura Abbott <labbott@...hat.com>,
        Mark Rutland <mark.rutland@....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 <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 12/17] arm64: reserve x18 from general allocation with SCS

On Thu, Oct 31, 2019 at 9:47 AM <samitolvanen@...gle.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.

Inline/out-of-line assembly will also need to be careful not to
accidentally overwrite the current task's shadow stack pointer.

Without automated checking for that, we'll have to rely on a
"gentlemen's promise" for code reviewers, though we'd likely hit such
a regression with Android+mainline.

Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>

>
> Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>
> ---
>  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
>


-- 
Thanks,
~Nick Desaulniers