[PATCH v15 22/27] riscv: enable kernel access to shadow stack memory via FWFT sbi call

[Alexandre Ghiti]

Hi Deepak,

On 03/05/2025 01:30, Deepak Gupta wrote:
> Kernel will have to perform shadow stack operations on user shadow stack.
> Like during signal delivery and sigreturn, shadow stack token must be
> created and validated respectively. Thus shadow stack access for kernel
> must be enabled.
>
> In future when kernel shadow stacks are enabled for linux kernel, it must
> be enabled as early as possible for better coverage and prevent imbalance
> between regular stack and shadow stack. After `relocate_enable_mmu` has
> been done, this is as early as possible it can enabled.
>
> Reviewed-by: Zong Li <zong.li at sifive.com>
> Signed-off-by: Deepak Gupta <debug at rivosinc.com>
> ---
>   arch/riscv/kernel/asm-offsets.c |  4 ++++
>   arch/riscv/kernel/head.S        | 27 +++++++++++++++++++++++++++
>   2 files changed, 31 insertions(+)
>
> diff --git a/arch/riscv/kernel/asm-offsets.c b/arch/riscv/kernel/asm-offsets.c
> index f33945432f8f..7ab41f01aa17 100644
> --- a/arch/riscv/kernel/asm-offsets.c
> +++ b/arch/riscv/kernel/asm-offsets.c
> @@ -514,4 +514,8 @@ void asm_offsets(void)
>   	DEFINE(FREGS_A6,	    offsetof(struct __arch_ftrace_regs, a6));
>   	DEFINE(FREGS_A7,	    offsetof(struct __arch_ftrace_regs, a7));
>   #endif
> +	DEFINE(SBI_EXT_FWFT, SBI_EXT_FWFT);
> +	DEFINE(SBI_EXT_FWFT_SET, SBI_EXT_FWFT_SET);
> +	DEFINE(SBI_FWFT_SHADOW_STACK, SBI_FWFT_SHADOW_STACK);
> +	DEFINE(SBI_FWFT_SET_FLAG_LOCK, SBI_FWFT_SET_FLAG_LOCK);


kernel test robot reported errors when !RV64 and !SBI, the following 
diff fixes it:

diff --git a/arch/riscv/kernel/asm-offsets.c 
b/arch/riscv/kernel/asm-offsets.c
index 7fc085d27ca79..3aa5f56a84e9a 100644
--- a/arch/riscv/kernel/asm-offsets.c
+++ b/arch/riscv/kernel/asm-offsets.c
@@ -532,8 +532,10 @@ void asm_offsets(void)
         DEFINE(FREGS_A6,            offsetof(struct __arch_ftrace_regs, 
a6));
         DEFINE(FREGS_A7,            offsetof(struct __arch_ftrace_regs, 
a7));
  #endif
+#ifdef CONFIG_RISCV_SBI
         DEFINE(SBI_EXT_FWFT, SBI_EXT_FWFT);
         DEFINE(SBI_EXT_FWFT_SET, SBI_EXT_FWFT_SET);
         DEFINE(SBI_FWFT_SHADOW_STACK, SBI_FWFT_SHADOW_STACK);
         DEFINE(SBI_FWFT_SET_FLAG_LOCK, SBI_FWFT_SET_FLAG_LOCK);
+#endif
  }

No need to resend the whole series, I'll squash it.

Thanks,

Alex


>   }
> diff --git a/arch/riscv/kernel/head.S b/arch/riscv/kernel/head.S
> index 356d5397b2a2..7eae9a172351 100644
> --- a/arch/riscv/kernel/head.S
> +++ b/arch/riscv/kernel/head.S
> @@ -15,6 +15,7 @@
>   #include <asm/image.h>
>   #include <asm/scs.h>
>   #include <asm/xip_fixup.h>
> +#include <asm/usercfi.h>
>   #include "efi-header.S"
>   
>   __HEAD
> @@ -164,6 +165,19 @@ secondary_start_sbi:
>   	call relocate_enable_mmu
>   #endif
>   	call .Lsetup_trap_vector
> +#if defined(CONFIG_RISCV_SBI) && defined(CONFIG_RISCV_USER_CFI)
> +	li a7, SBI_EXT_FWFT
> +	li a6, SBI_EXT_FWFT_SET
> +	li a0, SBI_FWFT_SHADOW_STACK
> +	li a1, 1 /* enable supervisor to access shadow stack access */
> +	li a2, SBI_FWFT_SET_FLAG_LOCK
> +	ecall
> +	beqz a0, 1f
> +	la a1, riscv_nousercfi
> +	li a0, CMDLINE_DISABLE_RISCV_USERCFI_BCFI
> +	REG_S a0, (a1)
> +1:
> +#endif
>   	scs_load_current
>   	call smp_callin
>   #endif /* CONFIG_SMP */
> @@ -320,6 +334,19 @@ SYM_CODE_START(_start_kernel)
>   	la tp, init_task
>   	la sp, init_thread_union + THREAD_SIZE
>   	addi sp, sp, -PT_SIZE_ON_STACK
> +#if defined(CONFIG_RISCV_SBI) && defined(CONFIG_RISCV_USER_CFI)
> +	li a7, SBI_EXT_FWFT
> +	li a6, SBI_EXT_FWFT_SET
> +	li a0, SBI_FWFT_SHADOW_STACK
> +	li a1, 1 /* enable supervisor to access shadow stack access */
> +	li a2, SBI_FWFT_SET_FLAG_LOCK
> +	ecall
> +	beqz a0, 1f
> +	la a1, riscv_nousercfi
> +	li a0, CMDLINE_DISABLE_RISCV_USERCFI_BCFI
> +	REG_S a0, (a1)
> +1:
> +#endif
>   	scs_load_current
>   
>   #ifdef CONFIG_KASAN
>