Le 01/06/2022 à 07:48, Rohan McLure a écrit :
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>
> Clears user state in gprs to reduce the influence of user registers on
> speculation within kernel syscall handlers.
>
> Remove conditional branches on result of `syscall_exit_prepare` to
> restore non-volatile gprs, as these registers are always cleared and
> hence always must be restored.
Did you measure the implied performance loss ? That must be heavy.
In patch 2 it is said that clearing registers is optional. I can't see
it as an option here. Some PPC32 are not impacted by speculative
problems, could we activate that big hammer only when required ?
>
> Signed-off-by: Rohan McLure
> ---
> arch/powerpc/kernel/interrupt_64.S | 12 +---
> 1 file changed, 9 insertions(+), 3 deletions(-)
>
> diff --git a/arch/powerpc/kernel/interrupt_64.S
> b/arch/powerpc/kernel/interrupt_64.S
> index b11c2bd84827..e601ed999798 100644
> --- a/arch/powerpc/kernel/interrupt_64.S
> +++ b/arch/powerpc/kernel/interrupt_64.S
> @@ -108,6 +108,9 @@ END_FTR_SECTION_IFSET(CPU_FTR_HAS_PPR)
> * but this is the best we can do.
> */
>
> + ZERO_GPRS(5, 12)
> + ZERO_NVGPRS()
> +
> /* Calling convention has r3 = orig r0, r4 = regs */
> mr r3,r0
> bl system_call_exception
> @@ -138,6 +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
>
> @@ -180,7 +184,6 @@ END_FTR_SECTION_IFSET(CPU_FTR_HAS_PPR)
> ld r4,_LINK(r1)
> ld r5,_XER(r1)
>
> - REST_NVGPRS(r1)
> ld r0,GPR0(r1)
> mtcrr2
> mtctr r3
> @@ -308,6 +311,9 @@ END_BTB_FLUSH_SECTION
> wrteei 1
> #endif
>
> + ZERO_GPRS(5, 12)
> + ZERO_NVGPRS()
> +
> /* Calling convention has r3 = orig r0, r4 = regs */
> mr r3,r0
> bl system_call_exception
> @@ -350,6 +356,7 @@ 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 */
> @@ -377,7 +384,6 @@ 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)
> @@ -445,7 +451,7 @@ _ASM_NOKPROBE_SYMBOL(interrupt_return_\srr\()_user)
> bl interrupt_exit_user_prepare
> cmpdi r3,0
> bne-.Lrestore_nvgprs_\srr
> -.Lrestore_nvgprs_\srr\()_cont:
> + .Lrestore_nvgprs_\srr\()_cont:
> std r1,PACA_EXIT_SAVE_R1(r13) /* save r1 for restart */
> #ifdef CONFIG_PPC_BOOK3S
> .Linterrupt_return_\srr\()_user_rst_start:
> --
> 2.34.1
>
