linux-kernel - Re: [PATCH 13/18] arm64: preserve x18 when CPU is suspended

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Message-ID: <CAKwvOd=7g2zbGpL41KC=VgapTYYd7-XqFxf+WQUyHVVJSMq=5A@mail.gmail.com>
Date:   Fri, 18 Oct 2019 09:49:22 -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>,
        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>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        kernel-hardening@...ts.openwall.com,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 13/18] arm64: preserve x18 when CPU is suspended

On Fri, Oct 18, 2019 at 9:11 AM Sami Tolvanen <samitolvanen@...gle.com> wrote:
>
> Don't lose the current task's shadow stack when the CPU is suspended.
>
> Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>
> ---
>  arch/arm64/mm/proc.S | 6 ++++++
>  1 file changed, 6 insertions(+)
>
> diff --git a/arch/arm64/mm/proc.S b/arch/arm64/mm/proc.S
> index fdabf40a83c8..9a8bd4bc8549 100644
> --- a/arch/arm64/mm/proc.S
> +++ b/arch/arm64/mm/proc.S
> @@ -73,6 +73,9 @@ alternative_endif
>         stp     x8, x9, [x0, #48]
>         stp     x10, x11, [x0, #64]
>         stp     x12, x13, [x0, #80]
> +#ifdef CONFIG_SHADOW_CALL_STACK
> +       stp     x18, xzr, [x0, #96]

Could this be a str/ldr of just x18 rather than stp/ldp of x18 +
garbage?  Maybe there's no real cost difference, or some kind of
alignment invariant?

> +#endif
>         ret
>  ENDPROC(cpu_do_suspend)
>
> @@ -89,6 +92,9 @@ ENTRY(cpu_do_resume)
>         ldp     x9, x10, [x0, #48]
>         ldp     x11, x12, [x0, #64]
>         ldp     x13, x14, [x0, #80]
> +#ifdef CONFIG_SHADOW_CALL_STACK
> +       ldp     x18, x19, [x0, #96]
> +#endif
>         msr     tpidr_el0, x2
>         msr     tpidrro_el0, x3
>         msr     contextidr_el1, x4
> --
> 2.23.0.866.gb869b98d4c-goog
>


-- 
Thanks,
~Nick Desaulniers