Re: [PATCH v6 10/15] arm64: preserve x18 when CPU is suspended

[Will Deacon]

On Fri, Dec 06, 2019 at 02:13:46PM -0800, Sami Tolvanen wrote:
> Don't lose the current task's shadow stack when the CPU is suspended.
> 
> Signed-off-by: Sami Tolvanen <samitolvanen@xxxxxxxxxx>
> Reviewed-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
> Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
> Reviewed-by: Mark Rutland <mark.rutland@xxxxxxx>
> ---
>  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]

Mumble, mumble, spectre-v4.

But I think it's fairly hopeless trying to fix that everywhere it crops up,
so:

Acked-by: Will Deacon <will@xxxxxxxxxx>

Will