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

[Deepak Gupta <debug@rivosinc.com>]

On Thu, May 15, 2025 at 09:10:08AM +0200, Alexandre Ghiti wrote:
>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@sifive.com>
>>Signed-off-by: Deepak Gupta <debug@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.
>
>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
>>