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

Mark Rutland
2019-11-15T15:21:02Z

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.