linux-kernel - Re: [PATCH v5 10/14] arm64: preserve x18 when CPU is suspended

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Message-ID: <CAKwvOdkGUn+X2HCnV7zM8ruCPYBsRi_UD8JY4VW4FbuOam8Pmg@mail.gmail.com>
Date:   Wed, 6 Nov 2019 12:39:14 -0800
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>,
        Marc Zyngier <maz@...nel.org>, 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 v5 10/14] arm64: preserve x18 when CPU is suspended

On Tue, Nov 5, 2019 at 3:56 PM 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>
> Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>

Re-LGTM

> Reviewed-by: Kees Cook <keescook@...omium.org>
> ---
>  arch/arm64/include/asm/suspend.h |  2 +-
>  arch/arm64/mm/proc.S             | 14 ++++++++++++++
>  2 files changed, 15 insertions(+), 1 deletion(-)
>
> diff --git a/arch/arm64/include/asm/suspend.h b/arch/arm64/include/asm/suspend.h
> index 8939c87c4dce..0cde2f473971 100644
> --- a/arch/arm64/include/asm/suspend.h
> +++ b/arch/arm64/include/asm/suspend.h
> @@ -2,7 +2,7 @@
>  #ifndef __ASM_SUSPEND_H
>  #define __ASM_SUSPEND_H
>
> -#define NR_CTX_REGS 12
> +#define NR_CTX_REGS 13
>  #define NR_CALLEE_SAVED_REGS 12
>
>  /*
> diff --git a/arch/arm64/mm/proc.S b/arch/arm64/mm/proc.S
> index fdabf40a83c8..5c8219c55948 100644
> --- a/arch/arm64/mm/proc.S
> +++ b/arch/arm64/mm/proc.S
> @@ -49,6 +49,8 @@
>   * cpu_do_suspend - save CPU registers context
>   *
>   * x0: virtual address of context pointer
> + *
> + * This must be kept in sync with struct cpu_suspend_ctx in <asm/suspend.h>.
>   */
>  ENTRY(cpu_do_suspend)
>         mrs     x2, tpidr_el0
> @@ -73,6 +75,11 @@ alternative_endif
>         stp     x8, x9, [x0, #48]
>         stp     x10, x11, [x0, #64]
>         stp     x12, x13, [x0, #80]
> +       /*
> +        * Save x18 as it may be used as a platform register, e.g. by shadow
> +        * call stack.
> +        */
> +       str     x18, [x0, #96]
>         ret
>  ENDPROC(cpu_do_suspend)
>
> @@ -89,6 +96,13 @@ ENTRY(cpu_do_resume)
>         ldp     x9, x10, [x0, #48]
>         ldp     x11, x12, [x0, #64]
>         ldp     x13, x14, [x0, #80]
> +       /*
> +        * Restore x18, as it may be used as a platform register, and clear
> +        * the buffer to minimize the risk of exposure when used for shadow
> +        * call stack.
> +        */
> +       ldr     x18, [x0, #96]
> +       str     xzr, [x0, #96]
>         msr     tpidr_el0, x2
>         msr     tpidrro_el0, x3
>         msr     contextidr_el1, x4
> --
> 2.24.0.rc1.363.gb1bccd3e3d-goog
>


-- 
Thanks,
~Nick Desaulniers