From 41213bf2ae6c936f51a79986b37f95da9ecbb970 Mon Sep 17 00:00:00 2001 From: Deepak Gupta Date: Sun, 25 Jan 2026 21:09:55 -0700 Subject: [PATCH] riscv: enable kernel access to shadow stack memory via the FWFT SBI call The kernel has to perform shadow stack operations on the user shadow stack. During signal delivery and sigreturn, the shadow stack token must be created and validated respectively. Thus shadow stack access for the kernel must be enabled. In the future, when kernel shadow stacks are enabled, they must be enabled as early as possible for better coverage and to prevent any imbalance between the regular stack and the shadow stack. After 'relocate_enable_mmu' has completed, this is the earliest that it can be enabled. Reviewed-by: Zong Li Signed-off-by: Deepak Gupta Tested-by: Andreas Korb # QEMU, custom CVA6 Tested-by: Valentin Haudiquet Link: https://patch.msgid.link/20251112-v5_user_cfi_series-v23-22-b55691eacf4f@rivosinc.com [pjw@kernel.org: updated to apply; cleaned up commit message] Signed-off-by: Paul Walmsley --- arch/riscv/kernel/asm-offsets.c | 6 ++++++ arch/riscv/kernel/head.S | 27 +++++++++++++++++++++++++++ 2 files changed, 33 insertions(+) diff --git a/arch/riscv/kernel/asm-offsets.c b/arch/riscv/kernel/asm-offsets.c index 8a2b2656cb2fd..af827448a609e 100644 --- a/arch/riscv/kernel/asm-offsets.c +++ b/arch/riscv/kernel/asm-offsets.c @@ -533,4 +533,10 @@ 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 } diff --git a/arch/riscv/kernel/head.S b/arch/riscv/kernel/head.S index bdf3352acf4cb..9c99c5ad6fe8a 100644 --- a/arch/riscv/kernel/head.S +++ b/arch/riscv/kernel/head.S @@ -15,6 +15,7 @@ #include #include #include +#include #include "efi-header.S" __HEAD @@ -170,6 +171,19 @@ 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 */ @@ -330,6 +344,19 @@ 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 -- 2.47.3