[PATCH 20/23] powerpc/64s: Clear/restore caller gprs in syscall interrupt/return

[Nicholas Piggin]

On Fri Sep 16, 2022 at 3:32 PM AEST, Rohan McLure wrote:
> Clear user state in gprs (assign to zero) to reduce the influence of user
> registers on speculation within kernel syscall handlers. Clears occur
> at the very beginning of the sc and scv 0 interrupt handlers, with
> restores occurring following the execution of the syscall handler.
>
> Signed-off-by: Rohan McLure <rmclure at linux.ibm.com>
> ---
> V1 -> V2: Update summary
> V2 -> V3: Remove erroneous summary paragraph on syscall_exit_prepare
> V3 -> V4: Use ZEROIZE instead of NULLIFY. Clear r0 also.
> V4 -> V5: Move to end of patch series.
> ---

Oh I would say change the title more like the interrupt patches to avoid
mentioning the restore/return because that's implicit.

e.g., "Clear user GPRs in syscall interrupt entry"

Could we have the one config option that does both this and the interrupt
zeroing?

Thanks,
Nick

>  arch/powerpc/kernel/interrupt_64.S | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
>
> diff --git a/arch/powerpc/kernel/interrupt_64.S b/arch/powerpc/kernel/interrupt_64.S
> index 16a1b44088e7..40147558e1a6 100644
> --- a/arch/powerpc/kernel/interrupt_64.S
> +++ b/arch/powerpc/kernel/interrupt_64.S
> @@ -70,7 +70,7 @@ _ASM_NOKPROBE_SYMBOL(system_call_vectored_\name)
>  	ld	r2,PACATOC(r13)
>  	mfcr	r12
>  	li	r11,0
> -	/* Can we avoid saving r3-r8 in common case? */
> +	/* Save syscall parameters in r3-r8 */
>  	SAVE_GPRS(3, 8, r1)
>  	/* Zero r9-r12, this should only be required when restoring all GPRs */
>  	std	r11,GPR9(r1)
> @@ -110,6 +110,7 @@ END_FTR_SECTION_IFSET(CPU_FTR_HAS_PPR)
>  	 * Zero user registers to prevent influencing speculative execution
>  	 * state of kernel code.
>  	 */
> +	ZEROIZE_GPR(0)
>  	ZEROIZE_GPRS(5, 12)
>  	ZEROIZE_NVGPRS()
>  	bl	system_call_exception
> @@ -140,6 +141,7 @@ BEGIN_FTR_SECTION
>  	HMT_MEDIUM_LOW
>  END_FTR_SECTION_IFSET(CPU_FTR_HAS_PPR)
>  
> +	REST_NVGPRS(r1)
>  	cmpdi	r3,0
>  	bne	.Lsyscall_vectored_\name\()_restore_regs
>  
> @@ -243,7 +245,7 @@ END_BTB_FLUSH_SECTION
>  	ld	r2,PACATOC(r13)
>  	mfcr	r12
>  	li	r11,0
> -	/* Can we avoid saving r3-r8 in common case? */
> +	/* Save syscall parameters in r3-r8 */
>  	SAVE_GPRS(3, 8, r1)
>  	/* Zero r9-r12, this should only be required when restoring all GPRs */
>  	std	r11,GPR9(r1)
> @@ -295,6 +297,7 @@ END_BTB_FLUSH_SECTION
>  	 * Zero user registers to prevent influencing speculative execution
>  	 * state of kernel code.
>  	 */
> +	ZEROIZE_GPR(0)
>  	ZEROIZE_GPRS(5, 12)
>  	ZEROIZE_NVGPRS()
>  	bl	system_call_exception
> @@ -337,6 +340,7 @@ BEGIN_FTR_SECTION
>  	stdcx.	r0,0,r1			/* to clear the reservation */
>  END_FTR_SECTION_IFCLR(CPU_FTR_STCX_CHECKS_ADDRESS)
>  
> +	REST_NVGPRS(r1)
>  	cmpdi	r3,0
>  	bne	.Lsyscall_restore_regs
>  	/* Zero volatile regs that may contain sensitive kernel data */
> @@ -364,7 +368,6 @@ END_FTR_SECTION_IFSET(CPU_FTR_HAS_PPR)
>  .Lsyscall_restore_regs:
>  	ld	r3,_CTR(r1)
>  	ld	r4,_XER(r1)
> -	REST_NVGPRS(r1)
>  	mtctr	r3
>  	mtspr	SPRN_XER,r4
>  	REST_GPR(0, r1)
> -- 
> 2.34.1