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.

Signed-off-by: Deepak Gupta <debug@rivosinc.com>
---
 arch/riscv/kernel/asm-offsets.c |  4 ++++
 arch/riscv/kernel/head.S        | 12 ++++++++++++
 2 files changed, 16 insertions(+)

diff --git a/arch/riscv/kernel/asm-offsets.c b/arch/riscv/kernel/asm-offsets.c
index 766bd33f10cb..a22ab8a41672 100644
--- a/arch/riscv/kernel/asm-offsets.c
+++ b/arch/riscv/kernel/asm-offsets.c
@@ -517,4 +517,8 @@ void asm_offsets(void)
 	DEFINE(FREGS_A6,	    offsetof(struct ftrace_regs, a6));
 	DEFINE(FREGS_A7,	    offsetof(struct 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);
 }
diff --git a/arch/riscv/kernel/head.S b/arch/riscv/kernel/head.S
index 356d5397b2a2..6244408ca917 100644
--- a/arch/riscv/kernel/head.S
+++ b/arch/riscv/kernel/head.S
@@ -164,6 +164,12 @@ secondary_start_sbi:
 	call relocate_enable_mmu
 #endif
 	call .Lsetup_trap_vector
+	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
 	scs_load_current
 	call smp_callin
 #endif /* CONFIG_SMP */
@@ -320,6 +326,12 @@ SYM_CODE_START(_start_kernel)
 	la tp, init_task
 	la sp, init_thread_union + THREAD_SIZE
 	addi sp, sp, -PT_SIZE_ON_STACK
+	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
 	scs_load_current
 
 #ifdef CONFIG_KASAN

-- 
2.45.0