linux-kernel - [RFC PATCH 08/12] riscv: dynamic (zicfiss) shadow call stack support

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Message-Id: <20240409061043.3269676-9-debug@rivosinc.com>
Date: Mon,  8 Apr 2024 23:10:39 -0700
From: Deepak Gupta <debug@...osinc.com>
To: linux-riscv@...ts.infradead.org,
	linux-kernel@...r.kernel.org,
	llvm@...ts.linux.dev
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Subject: [RFC PATCH 08/12] riscv: dynamic (zicfiss) shadow call stack support

Adding support for dynamic shadow call stack on riscv. zicfiss ISA extn.
enables protection for shadow stack against stray writes. 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_DYNAMIC_SCS is 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 | 47 +++++++++++++++++++++++++++++-------
 arch/riscv/kernel/entry.S    | 14 +++++------
 arch/riscv/kernel/head.S     |  4 +--
 4 files changed, 48 insertions(+), 19 deletions(-)

diff --git a/arch/riscv/include/asm/asm.h b/arch/riscv/include/asm/asm.h
index 776354895b81..0304978ea4e4 100644
--- a/arch/riscv/include/asm/asm.h
+++ b/arch/riscv/include/asm/asm.h
@@ -109,7 +109,7 @@
 	REG_L \dst, 0(\dst)
 .endm
 
-#ifdef CONFIG_SHADOW_CALL_STACK
+#if defined(CONFIG_SHADOW_CALL_STACK) && !defined(CONFIG_DYNAMIC_SCS)
 /* 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..14ef539922c2 100644
--- a/arch/riscv/include/asm/scs.h
+++ b/arch/riscv/include/asm/scs.h
@@ -9,46 +9,75 @@
 
 /* Load init_shadow_call_stack to gp. */
 .macro scs_load_init_stack
+#ifndef CONFIG_DYNAMIC_SCS
 	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_DYNAMIC_SCS
+	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_DYNAMIC_SCS
+	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_DYNAMIC_SCS
+	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_DYNAMIC_SCS
+#define arch_scs_store(ss_addr, magic_val)	\
+	asm volatile ("ssamoswap.d %0, %2, %1"	\
+					: "=r" (magic_val), "+A" (*ss_addr)	\
+					: "r" (magic_val)	\
+					: "memory")
+#else
+#define arch_scs_store(ss_addr, magic_val)
+#endif
+
 #endif /* _ASM_SCS_H */
diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S
index a35050a3e0ea..0262b46ab064 100644
--- a/arch/riscv/kernel/entry.S
+++ b/arch/riscv/kernel/entry.S
@@ -81,7 +81,7 @@ 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
@@ -135,7 +135,7 @@ 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
@@ -252,8 +252,8 @@ 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
@@ -263,7 +263,7 @@ 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
@@ -305,7 +305,7 @@ SYM_FUNC_START(__switch_to)
 	REG_S s10, TASK_THREAD_S10_RA(a3)
 	REG_S s11, TASK_THREAD_S11_RA(a3)
 	/* Save the kernel shadow call stack pointer */
-	scs_save_current
+	scs_save_current t0
 	/* Restore context from next->thread */
 	REG_L ra,  TASK_THREAD_RA_RA(a4)
 	REG_L sp,  TASK_THREAD_SP_RA(a4)
@@ -324,7 +324,7 @@ 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 6c311517c3b5..bc248c137c90 100644
--- a/arch/riscv/kernel/head.S
+++ b/arch/riscv/kernel/head.S
@@ -164,7 +164,7 @@ secondary_start_sbi:
 	call relocate_enable_mmu
 #endif
 	call .Lsetup_trap_vector
-	scs_load_current
+	scs_load_current t0
 	lui t2, 0x1
 	tail smp_callin
 #endif /* CONFIG_SMP */
@@ -313,7 +313,7 @@ SYM_CODE_START(_start_kernel)
 	la tp, init_task
 	la sp, init_thread_union + THREAD_SIZE
 	addi sp, sp, -PT_SIZE_ON_STACK
-	scs_load_current
+	scs_load_current t0
 
 #ifdef CONFIG_KASAN
 	call kasan_early_init
-- 
2.43.2