Re: [PATCH v6 14/15] arm64: implement Shadow Call Stack

[Will Deacon]

On Fri, Dec 06, 2019 at 02:13:50PM -0800, Sami Tolvanen wrote:
> This change implements shadow stack switching, initial SCS set-up,
> and interrupt shadow stacks for arm64.
> 
> Signed-off-by: Sami Tolvanen <samitolvanen@xxxxxxxxxx>
> Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
>  arch/arm64/Kconfig                   |  5 ++++
>  arch/arm64/include/asm/scs.h         | 37 +++++++++++++++++++++++++
>  arch/arm64/include/asm/thread_info.h |  3 +++
>  arch/arm64/kernel/Makefile           |  1 +
>  arch/arm64/kernel/asm-offsets.c      |  3 +++
>  arch/arm64/kernel/entry.S            | 31 +++++++++++++++++++--
>  arch/arm64/kernel/head.S             |  9 +++++++
>  arch/arm64/kernel/irq.c              |  2 ++
>  arch/arm64/kernel/process.c          |  2 ++
>  arch/arm64/kernel/scs.c              | 40 ++++++++++++++++++++++++++++
>  arch/arm64/kernel/smp.c              |  4 +++
>  11 files changed, 135 insertions(+), 2 deletions(-)
>  create mode 100644 arch/arm64/include/asm/scs.h
>  create mode 100644 arch/arm64/kernel/scs.c

[...]

> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> index 583f71abbe98..7aa2d366b2df 100644
> --- a/arch/arm64/kernel/entry.S
> +++ b/arch/arm64/kernel/entry.S
> @@ -172,6 +172,10 @@ alternative_cb_end
>  
>  	apply_ssbd 1, x22, x23
>  
> +#ifdef CONFIG_SHADOW_CALL_STACK
> +	ldr	x18, [tsk, #TSK_TI_SCS]		// Restore shadow call stack
> +	str	xzr, [tsk, #TSK_TI_SCS]		// Limit visibility of saved SCS
> +#endif
>  	.else
>  	add	x21, sp, #S_FRAME_SIZE
>  	get_current_task tsk
> @@ -280,6 +284,12 @@ alternative_else_nop_endif
>  	ct_user_enter
>  	.endif
>  
> +#ifdef CONFIG_SHADOW_CALL_STACK
> +	.if	\el == 0
> +	str	x18, [tsk, #TSK_TI_SCS]		// Save shadow call stack
> +	.endif
> +#endif
> +
>  #ifdef CONFIG_ARM64_SW_TTBR0_PAN
>  	/*
>  	 * Restore access to TTBR0_EL1. If returning to EL0, no need for SPSR
> @@ -385,6 +395,9 @@ alternative_insn eret, nop, ARM64_UNMAP_KERNEL_AT_EL0
>  
>  	.macro	irq_stack_entry
>  	mov	x19, sp			// preserve the original sp
> +#ifdef CONFIG_SHADOW_CALL_STACK
> +	mov	x20, x18		// preserve the original shadow stack
> +#endif

Hmm, not sure about corrupting x20 here. Doesn't it hold the PMR value from
kernel_entry?

Rest of the patch looks ok, but I'll do a proper review when it's closer to
being merged as we've got a bunch of other entry changes in the pipeline.

Will