[PATCH 21/23] powerpc/64: Add INTERRUPT_SANITIZE_REGISTERS Kconfig

[Nicholas Piggin]

On Fri Sep 16, 2022 at 3:32 PM AEST, Rohan McLure wrote:
> Add Kconfig option for enabling clearing of registers on arrival in an
> interrupt handler. This reduces the speculation influence of registers
> on kernel internals. The option will be consumed by 64-bit systems that
> feature speculation and wish to implement this mitigation.
>
> This patch only introduces the Kconfig option, no actual mitigations.
>
> The primary overhead of this mitigation lies in an increased number of
> registers that must be saved and restored by interrupt handlers on
> Book3S systems. Enable by default on Book3E systems, which prior to
> this patch eagerly save and restore register state, meaning that the
> mitigation when implemented will have minimal overhead.
>
> Signed-off-by: Rohan McLure <rmclure at linux.ibm.com>

Thanks for adding this. Maybe I'm being sentimental and we might end
up just simplifying code by always doing it... for now I'm happier to
have the option.

Acked-by: Nicholas Piggin <npiggin at gmail.com>

> ---
> V4 -> V5: New patch
> ---
>  arch/powerpc/Kconfig | 9 +++++++++
>  1 file changed, 9 insertions(+)
>
> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
> index ef6c83e79c9b..a643ebd83349 100644
> --- a/arch/powerpc/Kconfig
> +++ b/arch/powerpc/Kconfig
> @@ -528,6 +528,15 @@ config HOTPLUG_CPU
>  
>  	  Say N if you are unsure.
>  
> +config INTERRUPT_SANITIZE_REGISTERS
> +	bool "Clear gprs on interrupt arrival"
> +	depends on PPC64 && ARCH_HAS_SYSCALL_WRAPPER
> +	default PPC_BOOK3E_64
> +	help
> +	  Reduce the influence of user register state on interrupt handlers and
> +	  syscalls through clearing user state from registers before handling
> +	  the exception.
> +
>  config PPC_QUEUED_SPINLOCKS
>  	bool "Queued spinlocks" if EXPERT
>  	depends on SMP
> -- 
> 2.34.1