[PATCH v12 22/28] riscv: enable kernel access to shadow stack memory via FWFT sbi call

Deepak Gupta
2025-03-14T21:40:25Z

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 | 12 ++++++++++++
2 files changed, 16 insertions(+)

diff --git a/arch/riscv/kernel/asm-offsets.c b/arch/riscv/kernel/asm-offsets.c
index 0c188aaf3925..21f99d5757b6 100644
--- a/arch/riscv/kernel/asm-offsets.c
+++ b/arch/riscv/kernel/asm-offsets.c
@@ -515,4 +515,8 @@ 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);
}
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

--
2.34.1