Message-Id: <20250724-riscv_kcfi-v1-9-04b8fa44c98c@rivosinc.com>
Date: Thu, 24 Jul 2025 16:37:02 -0700
From: Deepak Gupta <debug@...osinc.com>
To: Paul Walmsley <paul.walmsley@...ive.com>, 
 Palmer Dabbelt <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>, 
 Alexandre Ghiti <alex@...ti.fr>, Masahiro Yamada <masahiroy@...nel.org>, 
 Nathan Chancellor <nathan@...nel.org>, 
 Nicolas Schier <nicolas.schier@...ux.dev>, 
 Andrew Morton <akpm@...ux-foundation.org>, 
 David Hildenbrand <david@...hat.com>, 
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, 
 "Liam R. Howlett" <Liam.Howlett@...cle.com>, 
 Vlastimil Babka <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>, 
 Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>, 
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 Bill Wendling <morbo@...gle.com>, Monk Chiang <monk.chiang@...ive.com>, 
 Kito Cheng <kito.cheng@...ive.com>, Justin Stitt <justinstitt@...gle.com>
Cc: linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org, 
 linux-kbuild@...r.kernel.org, linux-mm@...ck.org, llvm@...ts.linux.dev, 
 rick.p.edgecombe@...el.com, broonie@...nel.org, cleger@...osinc.com, 
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 bjorn@...osinc.com, fweimer@...hat.com, jeffreyalaw@...il.com, 
 heinrich.schuchardt@...onical.com, andrew@...ive.com, ved@...osinc.com, 
 Deepak Gupta <debug@...osinc.com>
Subject: [PATCH 09/11] riscv: scs: add hardware shadow stack support to scs

Adding support for hardware support for shadow call stack on riscv. This
patch enables scs_* macros to use zicfiss shadow stack pointer (CSR_SSP)
instead of relying on `gp`.

Since zicfiss based shadow stack needs to have correct encoding set in PTE
init shadow stack can't be established too early. It has to be setup after
`setup_vm` is called. Thus `scs_load_init_stack` is noped out if
CONFIG_ARCH_HAS_KERNEL_SHADOW_STACK is not selected.

Adds `arch_scs_store` that can be used in generic scs magic store routine.

Signed-off-by: Deepak Gupta <debug@...osinc.com>
---
 arch/riscv/include/asm/asm.h |  2 +-
 arch/riscv/include/asm/scs.h | 48 +++++++++++++++++++++++++++++++++++---------
 arch/riscv/kernel/entry.S    | 14 ++++++-------
 arch/riscv/kernel/head.S     |  4 ++--
 4 files changed, 49 insertions(+), 19 deletions(-)

diff --git a/arch/riscv/include/asm/asm.h b/arch/riscv/include/asm/asm.h
index a8a2af6dfe9d..256aff523dd4 100644
--- a/arch/riscv/include/asm/asm.h
+++ b/arch/riscv/include/asm/asm.h
@@ -110,7 +110,7 @@
 	REG_L \dst, 0(\dst)
 .endm
 
-#ifdef CONFIG_SHADOW_CALL_STACK
+#if defined(CONFIG_SHADOW_CALL_STACK) && !defined(CONFIG_ARCH_HAS_KERNEL_SHADOW_STACK)
 /* gp is used as the shadow call stack pointer instead */
 .macro load_global_pointer
 .endm
diff --git a/arch/riscv/include/asm/scs.h b/arch/riscv/include/asm/scs.h
index 0e45db78b24b..e70e6ef14bc5 100644
--- a/arch/riscv/include/asm/scs.h
+++ b/arch/riscv/include/asm/scs.h
@@ -9,46 +9,76 @@
 
 /* Load init_shadow_call_stack to gp. */
 .macro scs_load_init_stack
+#ifndef CONFIG_ARCH_HAS_KERNEL_SHADOW_STACK
 	la	gp, init_shadow_call_stack
 	XIP_FIXUP_OFFSET gp
+#endif
 .endm
 
 /* Load the per-CPU IRQ shadow call stack to gp. */
-.macro scs_load_irq_stack tmp
+.macro scs_load_irq_stack tmp tmp1
+#ifdef CONFIG_ARCH_HAS_KERNEL_SHADOW_STACK
+	load_per_cpu \tmp1, irq_shadow_call_stack_ptr, \tmp
+	li \tmp, 4096
+	add \tmp, \tmp, \tmp1
+	csrw CSR_SSP, \tmp
+#else
 	load_per_cpu gp, irq_shadow_call_stack_ptr, \tmp
+#endif
 .endm
 
 /* Load task_scs_sp(current) to gp. */
-.macro scs_load_current
+.macro scs_load_current tmp
+#ifdef CONFIG_ARCH_HAS_KERNEL_SHADOW_STACK
+	REG_L	\tmp, TASK_TI_SCS_SP(tp)
+	csrw CSR_SSP, \tmp
+#else
 	REG_L	gp, TASK_TI_SCS_SP(tp)
+#endif
 .endm
 
 /* Load task_scs_sp(current) to gp, but only if tp has changed. */
-.macro scs_load_current_if_task_changed prev
+.macro scs_load_current_if_task_changed prev tmp
 	beq	\prev, tp, _skip_scs
-	scs_load_current
+	scs_load_current \tmp
 _skip_scs:
 .endm
 
 /* Save gp to task_scs_sp(current). */
-.macro scs_save_current
+.macro scs_save_current tmp
+#ifdef CONFIG_ARCH_HAS_KERNEL_SHADOW_STACK
+	csrr \tmp, CSR_SSP
+	REG_S	\tmp, TASK_TI_SCS_SP(tp)
+#else
 	REG_S	gp, TASK_TI_SCS_SP(tp)
+#endif
 .endm
 
 #else /* CONFIG_SHADOW_CALL_STACK */
 
 .macro scs_load_init_stack
 .endm
-.macro scs_load_irq_stack tmp
+.macro scs_load_irq_stack tmp tmp1
 .endm
-.macro scs_load_current
+.macro scs_load_current tmp
 .endm
-.macro scs_load_current_if_task_changed prev
+.macro scs_load_current_if_task_changed prev tmp
 .endm
-.macro scs_save_current
+.macro scs_save_current tmp
 .endm
 
 #endif /* CONFIG_SHADOW_CALL_STACK */
 #endif /* __ASSEMBLY__ */
 
+#ifdef CONFIG_ARCH_HAS_KERNEL_SHADOW_STACK
+#define arch_scs_store(ss_addr, magic_val)	do {				\
+	asm volatile ("ssamoswap.d %0, %2, %1"					\
+					: "=r" (magic_val), "+A" (*ss_addr)	\
+					: "r" (magic_val)			\
+					: "memory");				\
+	} while (0)
+#else
+#define arch_scs_store(ss_addr, magic_val) do {} while (0)
+#endif
+
 #endif /* _ASM_SCS_H */
diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S
index 3f0890b9c0b9..800a5ab763af 100644
--- a/arch/riscv/kernel/entry.S
+++ b/arch/riscv/kernel/entry.S
@@ -199,7 +199,7 @@ SYM_CODE_START(handle_exception)
 	load_global_pointer
 
 	/* Load the kernel shadow call stack pointer if coming from userspace */
-	scs_load_current_if_task_changed s5
+	scs_load_current_if_task_changed s5 t0
 
 #ifdef CONFIG_RISCV_ISA_V_PREEMPTIVE
 	move a0, sp
@@ -260,7 +260,7 @@ SYM_CODE_START_NOALIGN(ret_from_exception)
 	REG_S s0, TASK_TI_KERNEL_SP(tp)
 
 	/* Save the kernel shadow call stack pointer */
-	scs_save_current
+	scs_save_current t0
 
 	/*
 	 * Save TP into the scratch register , so we can find the kernel data
@@ -382,8 +382,8 @@ SYM_FUNC_START(call_on_irq_stack)
 	addi	s0, sp, STACKFRAME_SIZE_ON_STACK
 
 	/* Switch to the per-CPU shadow call stack */
-	scs_save_current
-	scs_load_irq_stack t0
+	scs_save_current t0
+	scs_load_irq_stack t0 t1
 
 	/* Switch to the per-CPU IRQ stack and call the handler */
 	load_per_cpu t0, irq_stack_ptr, t1
@@ -393,7 +393,7 @@ SYM_FUNC_START(call_on_irq_stack)
 	jalr	a1
 
 	/* Switch back to the thread shadow call stack */
-	scs_load_current
+	scs_load_current t0
 
 	/* Switch back to the thread stack and restore ra and s0 */
 	addi	sp, s0, -STACKFRAME_SIZE_ON_STACK
@@ -440,7 +440,7 @@ SYM_FUNC_START(__switch_to)
 	REG_S s0, TASK_THREAD_SUM_RA(a3)
 
 	/* Save the kernel shadow call stack pointer */
-	scs_save_current
+	scs_save_current t0
 	/* Restore context from next->thread */
 	REG_L s0,  TASK_THREAD_SUM_RA(a4)
 	li    s1,  SR_SUM
@@ -463,7 +463,7 @@ SYM_FUNC_START(__switch_to)
 	/* The offset of thread_info in task_struct is zero. */
 	move tp, a1
 	/* Switch to the next shadow call stack */
-	scs_load_current
+	scs_load_current t0
 	ret
 SYM_FUNC_END(__switch_to)
 
diff --git a/arch/riscv/kernel/head.S b/arch/riscv/kernel/head.S
index 59af044bf85c..366e15a9280a 100644
--- a/arch/riscv/kernel/head.S
+++ b/arch/riscv/kernel/head.S
@@ -184,7 +184,7 @@ secondary_start_sbi:
 	REG_S a0, (a1)
 1:
 #endif
-	scs_load_current
+	scs_load_current t0
 
 #if defined(CONFIG_RISCV_SBI) && defined(CONFIG_RISCV_KERNEL_CFI)
 	li a7, SBI_EXT_FWFT
@@ -367,7 +367,7 @@ SYM_CODE_START(_start_kernel)
 	REG_S a0, (a1)
 1:
 #endif
-	scs_load_current
+	scs_load_current t0
 
 #if defined(CONFIG_RISCV_SBI) && defined(CONFIG_RISCV_KERNEL_CFI)
 	li a7, SBI_EXT_FWFT

-- 
2.43.0

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