[PATCH 3/6] powerpc: Make syscalls save and restore gprs

[Christophe Leroy]

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 <rmclure at linux.ibm.com>
> ---
>   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)
>          mtcr    r2
>          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
>