Subject: Re: [PATCH v2 3/4] powerpc/64e: Clear gprs on interrupt routine entry on Book3E

From: "Nicholas Piggin" <[email-masked]>
Date: Mon, 28 Nov 2022 12:02:41 +1000

On Mon Nov 7, 2022 at 1:32 PM AEST, Rohan McLure wrote:
> Zero GPRS r14-r31 on entry into the kernel for interrupt sources to
> limit influence of user-space values in potential speculation gadgets.
> Prior to this commit, all other GPRS are reassigned during the common
> prologue to interrupt handlers and so need not be zeroised explicitly.
>
> This may be done safely, without loss of register state prior to the
> interrupt, as the common prologue saves the initial values of
> non-volatiles, which are unconditionally restored in interrupt_64.S.

In the case of ret_from_crit_except and ret_from_mc_except, it looks
like those are restored by ret_from_level_except, so that's fine.
And fast_interrupt_return you added NVGPRS restore in the previous
patch too.

Maybe actually you could move that interrupt_64.h code that applies to
both 64s and 64e in patch 1. So then the 64s/e enablement patches are
independent and apply to exactly that subarch.

But code-wise I think this looks good.

Reviewed-by: Nicholas Piggin <[email-masked]>

> Mitigation defaults to enabled by INTERRUPT_SANITIZE_REGISTERS.
>
> Signed-off-by: Rohan McLure <[email-masked]>
> ---
> Resubmitting patches as their own series after v6 partially merged:
> Link: https://lore.kernel.org/all/[email-masked]/t/
> ---
>  arch/powerpc/kernel/exceptions-64e.S | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/arch/powerpc/kernel/exceptions-64e.S b/arch/powerpc/kernel/exceptions-64e.S
> index 2f68fb2ee4fc..91d8019123c2 100644
> --- a/arch/powerpc/kernel/exceptions-64e.S
> +++ b/arch/powerpc/kernel/exceptions-64e.S
> @@ -358,6 +358,11 @@ ret_from_mc_except:
>  	std	r14,PACA_EXMC+EX_R14(r13);				    \
>  	std	r15,PACA_EXMC+EX_R15(r13)
>  
> +#ifdef CONFIG_INTERRUPT_SANITIZE_REGISTERS
> +#define SANITIZE_ZEROIZE_NVGPRS()	ZEROIZE_NVGPRS()
> +#else
> +#define SANITIZE_ZEROIZE_NVGPRS()
> +#endif

Could possibly share these macros.

>  
>  /* Core exception code for all exceptions except TLB misses. */
>  #define EXCEPTION_COMMON_LVL(n, scratch, excf)				    \
> @@ -394,7 +399,8 @@ exc_##n##_common:							    \
>  	std	r12,STACK_FRAME_OVERHEAD-16(r1); /* mark the frame */	    \
>  	std	r3,_TRAP(r1);		/* set trap number		*/  \
>  	std	r0,RESULT(r1);		/* clear regs->result */	    \
> -	SAVE_NVGPRS(r1);
> +	SAVE_NVGPRS(r1);						    \
> +	SANITIZE_ZEROIZE_NVGPRS();	/* minimise speculation influence */
>  
>  #define EXCEPTION_COMMON(n) \
>  	EXCEPTION_COMMON_LVL(n, SPRN_SPRG_GEN_SCRATCH, PACA_EXGEN)
> -- 
> 2.34.1