linux-kernel - Re: [PATCH v5 14/14] arm64: implement Shadow Call Stack

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Date:   Fri, 15 Nov 2019 15:20:48 +0000
From:   Mark Rutland <mark.rutland@....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>,
        Marc Zyngier <maz@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Jann Horn <jannh@...gle.com>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        clang-built-linux@...glegroups.com,
        kernel-hardening@...ts.openwall.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 14/14] arm64: implement Shadow Call Stack

On Tue, Nov 05, 2019 at 03:56:08PM -0800, Sami Tolvanen wrote:
> This change implements shadow stack switching, initial SCS set-up,
> and interrupt shadow stacks for arm64.

Each CPU also has an overflow stack, and two SDEI stacks, which should
presumably be given their own SCS. SDEI is effectively a software-NMI,
so it should almost certainly have the same treatement as IRQ.

> +static __always_inline void scs_save(struct task_struct *tsk)
> +{
> +	void *s;
> +
> +	asm volatile("mov %0, x18" : "=r" (s));
> +	task_set_scs(tsk, s);
> +}

An alternative would be to follow <asm/stack_pointer.h>, and have:

register unsigned long *current_scs_pointer asm ("x18");

static __always_inline void scs_save(struct task_struct *tsk)
{
	task_set_scs(tsk, current_scs_pointer);
}

... which would avoid the need for a temporary register where this is
used.

However, given we only use this in cpu_die(), having this as-is should
be fine. Maybe the asm volatile is preferable if we use this elsewhere,
so that we know we have a consistent snapshot that the compiler can't
reload, etc.

[...]

> @@ -409,6 +428,10 @@ alternative_insn eret, nop, ARM64_UNMAP_KERNEL_AT_EL0
>  	 */
>  	.macro	irq_stack_exit
>  	mov	sp, x19
> +#ifdef CONFIG_SHADOW_CALL_STACK
> +	/* x20 is also preserved */
> +	mov	x18, x20
> +#endif
>  	.endm

Can we please fold this comment into the one above, and have:

	/*
	 * The callee-saved regs (x19-x29) should be preserved between
	 * irq_stack_entry and irq_stack_exit.
	 */
	.macro irq_stack_exit
	mov     sp, x19
#ifdef CONFIG_SHADOW_CALL_STACK
	mov     x18, x20
#endif
	.endm

Thanks,
Mark.