Re: [PATCH v2 1/4] powerpc/64: Add INTERRUPT_SANITIZE_REGISTERS Kconfig

[Christophe Leroy <christophe.leroy@csgroup.eu>]

Le 07/11/2022 à 04:31, Rohan McLure a écrit :
> 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.

If that has to do with speculation, do we need a new Kconfig option ? 
Can't we use CONFIG_PPC_BARRIER_NOSPEC for that ?

> 
> 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.
> 
> Acked-by: Nicholas Piggin <npiggin@gmail.com>
> Signed-off-by: Rohan McLure <rmclure@linux.ibm.com>
> ---
> Resubmitting patches as their own series after v6 partially merged:
> Link: https://lore.kernel.org/all/166488988686.779920.13794870102696416283.b4-ty@ellerman.id.au/t/
> ---
>   arch/powerpc/Kconfig | 9 +++++++++
>   1 file changed, 9 insertions(+)
> 
> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
> index 2ca5418457ed..9d3d20c6f365 100644
> --- a/arch/powerpc/Kconfig
> +++ b/arch/powerpc/Kconfig
> @@ -529,6 +529,15 @@ 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