Merge patch series "riscv: SCS support"

Sami Tolvanen <samitolvanen@google.com> says:

This series adds Shadow Call Stack (SCS) support for RISC-V. SCS
uses compiler instrumentation to store return addresses in a
separate shadow stack to protect them against accidental or
malicious overwrites. More information about SCS can be found
here:

  https://clang.llvm.org/docs/ShadowCallStack.html

Patch 1 is from Deepak, and it simplifies VMAP_STACK overflow
handling by adding support for accessing per-CPU variables
directly in assembly. The patch is included in this series to
make IRQ stack switching cleaner with SCS, and I've simply
rebased it and fixed a couple of minor issues. Patch 2 uses this
functionality to clean up the stack switching by moving duplicate
code into a single function. On RISC-V, the compiler uses the
gp register for storing the current shadow call stack pointer,
which is incompatible with global pointer relaxation. Patch 3
moves global pointer loading into a macro that can be easily
disabled with SCS. Patch 4 implements SCS register loading and
switching, and allows the feature to be enabled, and patch 5 adds
separate per-CPU IRQ shadow call stacks when CONFIG_IRQ_STACKS is
enabled. Patch 6 fixes the backward-edge CFI test in lkdtm for
RISC-V.

Note that this series requires Clang 17. Earlier Clang versions
support SCS on RISC-V, but use the x18 register instead of gp,
which isn't ideal. gcc has SCS support for arm64, but I'm not
aware of plans to support RISC-V. Once the Zicfiss extension is
ratified, it's probably preferable to use hardware-backed shadow
stacks instead of SCS on hardware that supports the extension,
and we may want to consider implementing CONFIG_DYNAMIC_SCS to
patch between the implementation at runtime (similarly to the
arm64 implementation, which switches to SCS when hardware PAC
support isn't available).

* b4-shazam-merge:
  lkdtm: Fix CFI_BACKWARD on RISC-V
  riscv: Use separate IRQ shadow call stacks
  riscv: Implement Shadow Call Stack
  riscv: Move global pointer loading to a macro
  riscv: Deduplicate IRQ stack switching
  riscv: VMAP_STACK overflow detection thread-safe

Link: https://lore.kernel.org/r/20230927224757.1154247-8-samitolvanen@google.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

7 files changed, 112 insertions, 171 deletions

diff --git a/arch/riscv/kernel/asm-offsets.c b/arch/riscv/kernel/asm-offsets.c
index d6a75aac1d27..a03129f40c46 100644
--- a/arch/riscv/kernel/asm-offsets.c
+++ b/arch/riscv/kernel/asm-offsets.c
@@ -14,6 +14,7 @@
 #include <asm/thread_info.h>
 #include <asm/ptrace.h>
 #include <asm/cpu_ops_sbi.h>
+#include <asm/stacktrace.h>
 #include <asm/suspend.h>
 
 void asm_offsets(void);
@@ -38,7 +39,11 @@ void asm_offsets(void)
 	OFFSET(TASK_TI_PREEMPT_COUNT, task_struct, thread_info.preempt_count);
 	OFFSET(TASK_TI_KERNEL_SP, task_struct, thread_info.kernel_sp);
 	OFFSET(TASK_TI_USER_SP, task_struct, thread_info.user_sp);
+#ifdef CONFIG_SHADOW_CALL_STACK
+	OFFSET(TASK_TI_SCS_SP, task_struct, thread_info.scs_sp);
+#endif
 
+	OFFSET(TASK_TI_CPU_NUM, task_struct, thread_info.cpu);
 	OFFSET(TASK_THREAD_F0,  task_struct, thread.fstate.f[0]);
 	OFFSET(TASK_THREAD_F1,  task_struct, thread.fstate.f[1]);
 	OFFSET(TASK_THREAD_F2,  task_struct, thread.fstate.f[2]);
@@ -479,4 +484,8 @@ void asm_offsets(void)
 	OFFSET(KERNEL_MAP_VIRT_ADDR, kernel_mapping, virt_addr);
 	OFFSET(SBI_HART_BOOT_TASK_PTR_OFFSET, sbi_hart_boot_data, task_ptr);
 	OFFSET(SBI_HART_BOOT_STACK_PTR_OFFSET, sbi_hart_boot_data, stack_ptr);
+
+	DEFINE(STACKFRAME_SIZE_ON_STACK, ALIGN(sizeof(struct stackframe), STACK_ALIGN));
+	OFFSET(STACKFRAME_FP, stackframe, fp);
+	OFFSET(STACKFRAME_RA, stackframe, ra);
 }
diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S
index d7dd9030df3f..9f92c067f7e1 100644
--- a/arch/riscv/kernel/entry.S
+++ b/arch/riscv/kernel/entry.S
@@ -9,10 +9,13 @@
 
 #include <asm/asm.h>
 #include <asm/csr.h>
+#include <asm/scs.h>
 #include <asm/unistd.h>
+#include <asm/page.h>
 #include <asm/thread_info.h>
 #include <asm/asm-offsets.h>
 #include <asm/errata_list.h>
+#include <linux/sizes.h>
 
 	.section .irqentry.text, "ax"
 
@@ -75,10 +78,11 @@ _save_context:
 	csrw CSR_SCRATCH, x0
 
 	/* Load the global pointer */
-.option push
-.option norelax
-	la gp, __global_pointer$
-.option pop
+	load_global_pointer
+
+	/* Load the kernel shadow call stack pointer if coming from userspace */
+	scs_load_current_if_task_changed s5
+
 	move a0, sp /* pt_regs */
 	la ra, ret_from_exception
 
@@ -125,6 +129,9 @@ SYM_CODE_START_NOALIGN(ret_from_exception)
 	addi s0, sp, PT_SIZE_ON_STACK
 	REG_S s0, TASK_TI_KERNEL_SP(tp)
 
+	/* Save the kernel shadow call stack pointer */
+	scs_save_current
+
 	/*
 	 * Save TP into the scratch register , so we can find the kernel data
 	 * structures again.
@@ -172,67 +179,15 @@ SYM_CODE_END(ret_from_exception)
 
 #ifdef CONFIG_VMAP_STACK
 SYM_CODE_START_LOCAL(handle_kernel_stack_overflow)
-	/*
-	 * Takes the psuedo-spinlock for the shadow stack, in case multiple
-	 * harts are concurrently overflowing their kernel stacks.  We could
-	 * store any value here, but since we're overflowing the kernel stack
-	 * already we only have SP to use as a scratch register.  So we just
-	 * swap in the address of the spinlock, as that's definately non-zero.
-	 *
-	 * Pairs with a store_release in handle_bad_stack().
-	 */
-1:	la sp, spin_shadow_stack
-	REG_AMOSWAP_AQ sp, sp, (sp)
-	bnez sp, 1b
-
-	la sp, shadow_stack
-	addi sp, sp, SHADOW_OVERFLOW_STACK_SIZE
+	/* we reach here from kernel context, sscratch must be 0 */
+	csrrw x31, CSR_SCRATCH, x31
+	asm_per_cpu sp, overflow_stack, x31
+	li x31, OVERFLOW_STACK_SIZE
+	add sp, sp, x31
+	/* zero out x31 again and restore x31 */
+	xor x31, x31, x31
+	csrrw x31, CSR_SCRATCH, x31
 
-	//save caller register to shadow stack
-	addi sp, sp, -(PT_SIZE_ON_STACK)
-	REG_S x1,  PT_RA(sp)
-	REG_S x5,  PT_T0(sp)
-	REG_S x6,  PT_T1(sp)
-	REG_S x7,  PT_T2(sp)
-	REG_S x10, PT_A0(sp)
-	REG_S x11, PT_A1(sp)
-	REG_S x12, PT_A2(sp)
-	REG_S x13, PT_A3(sp)
-	REG_S x14, PT_A4(sp)
-	REG_S x15, PT_A5(sp)
-	REG_S x16, PT_A6(sp)
-	REG_S x17, PT_A7(sp)
-	REG_S x28, PT_T3(sp)
-	REG_S x29, PT_T4(sp)
-	REG_S x30, PT_T5(sp)
-	REG_S x31, PT_T6(sp)
-
-	la ra, restore_caller_reg
-	tail get_overflow_stack
-
-restore_caller_reg:
-	//save per-cpu overflow stack
-	REG_S a0, -8(sp)
-	//restore caller register from shadow_stack
-	REG_L x1,  PT_RA(sp)
-	REG_L x5,  PT_T0(sp)
-	REG_L x6,  PT_T1(sp)
-	REG_L x7,  PT_T2(sp)
-	REG_L x10, PT_A0(sp)
-	REG_L x11, PT_A1(sp)
-	REG_L x12, PT_A2(sp)
-	REG_L x13, PT_A3(sp)
-	REG_L x14, PT_A4(sp)
-	REG_L x15, PT_A5(sp)
-	REG_L x16, PT_A6(sp)
-	REG_L x17, PT_A7(sp)
-	REG_L x28, PT_T3(sp)
-	REG_L x29, PT_T4(sp)
-	REG_L x30, PT_T5(sp)
-	REG_L x31, PT_T6(sp)
-
-	//load per-cpu overflow stack
-	REG_L sp, -8(sp)
 	addi sp, sp, -(PT_SIZE_ON_STACK)
 
 	//save context to overflow stack
@@ -270,6 +225,43 @@ SYM_CODE_START(ret_from_fork)
 	tail syscall_exit_to_user_mode
 SYM_CODE_END(ret_from_fork)
 
+#ifdef CONFIG_IRQ_STACKS
+/*
+ * void call_on_irq_stack(struct pt_regs *regs,
+ * 		          void (*func)(struct pt_regs *));
+ *
+ * Calls func(regs) using the per-CPU IRQ stack.
+ */
+SYM_FUNC_START(call_on_irq_stack)
+	/* Create a frame record to save ra and s0 (fp) */
+	addi	sp, sp, -STACKFRAME_SIZE_ON_STACK
+	REG_S	ra, STACKFRAME_RA(sp)
+	REG_S	s0, STACKFRAME_FP(sp)
+	addi	s0, sp, STACKFRAME_SIZE_ON_STACK
+
+	/* Switch to the per-CPU shadow call stack */
+	scs_save_current
+	scs_load_irq_stack t0
+
+	/* Switch to the per-CPU IRQ stack and call the handler */
+	load_per_cpu t0, irq_stack_ptr, t1
+	li	t1, IRQ_STACK_SIZE
+	add	sp, t0, t1
+	jalr	a1
+
+	/* Switch back to the thread shadow call stack */
+	scs_load_current
+
+	/* Switch back to the thread stack and restore ra and s0 */
+	addi	sp, s0, -STACKFRAME_SIZE_ON_STACK
+	REG_L	ra, STACKFRAME_RA(sp)
+	REG_L	s0, STACKFRAME_FP(sp)
+	addi	sp, sp, STACKFRAME_SIZE_ON_STACK
+
+	ret
+SYM_FUNC_END(call_on_irq_stack)
+#endif /* CONFIG_IRQ_STACKS */
+
 /*
  * Integer register context switch
  * The callee-saved registers must be saved and restored.
@@ -299,6 +291,8 @@ SYM_FUNC_START(__switch_to)
 	REG_S s9,  TASK_THREAD_S9_RA(a3)
 	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
 	/* Restore context from next->thread */
 	REG_L ra,  TASK_THREAD_RA_RA(a4)
 	REG_L sp,  TASK_THREAD_SP_RA(a4)
@@ -316,6 +310,8 @@ SYM_FUNC_START(__switch_to)
 	REG_L s11, TASK_THREAD_S11_RA(a4)
 	/* The offset of thread_info in task_struct is zero. */
 	move tp, a1
+	/* Switch to the next shadow call stack */
+	scs_load_current
 	ret
 SYM_FUNC_END(__switch_to)
 
diff --git a/arch/riscv/kernel/head.S b/arch/riscv/kernel/head.S
index 3710ea5d160f..18f97ec0f7ed 100644
--- a/arch/riscv/kernel/head.S
+++ b/arch/riscv/kernel/head.S
@@ -14,6 +14,7 @@
 #include <asm/cpu_ops_sbi.h>
 #include <asm/hwcap.h>
 #include <asm/image.h>
+#include <asm/scs.h>
 #include <asm/xip_fixup.h>
 #include "efi-header.S"
 
@@ -110,10 +111,7 @@ relocate_enable_mmu:
 	csrw CSR_TVEC, a0
 
 	/* Reload the global pointer */
-.option push
-.option norelax
-	la gp, __global_pointer$
-.option pop
+	load_global_pointer
 
 	/*
 	 * Switch to kernel page tables.  A full fence is necessary in order to
@@ -134,10 +132,7 @@ secondary_start_sbi:
 	csrw CSR_IP, zero
 
 	/* Load the global pointer */
-	.option push
-	.option norelax
-		la gp, __global_pointer$
-	.option pop
+	load_global_pointer
 
 	/*
 	 * Disable FPU & VECTOR to detect illegal usage of
@@ -159,6 +154,7 @@ secondary_start_sbi:
 	XIP_FIXUP_OFFSET a3
 	add a3, a3, a1
 	REG_L sp, (a3)
+	scs_load_current
 
 .Lsecondary_start_common:
 
@@ -228,10 +224,7 @@ pmp_done:
 #endif /* CONFIG_RISCV_M_MODE */
 
 	/* Load the global pointer */
-.option push
-.option norelax
-	la gp, __global_pointer$
-.option pop
+	load_global_pointer
 
 	/*
 	 * Disable FPU & VECTOR to detect illegal usage of
@@ -298,6 +291,7 @@ clear_bss_done:
 	la sp, init_thread_union + THREAD_SIZE
 	XIP_FIXUP_OFFSET sp
 	addi sp, sp, -PT_SIZE_ON_STACK
+	scs_load_init_stack
 #ifdef CONFIG_BUILTIN_DTB
 	la a0, __dtb_start
 	XIP_FIXUP_OFFSET a0
@@ -316,6 +310,7 @@ clear_bss_done:
 	la tp, init_task
 	la sp, init_thread_union + THREAD_SIZE
 	addi sp, sp, -PT_SIZE_ON_STACK
+	scs_load_current
 
 #ifdef CONFIG_KASAN
 	call kasan_early_init
diff --git a/arch/riscv/kernel/irq.c b/arch/riscv/kernel/irq.c
index a8efa053c4a5..7bfea97ee7e7 100644
--- a/arch/riscv/kernel/irq.c
+++ b/arch/riscv/kernel/irq.c
@@ -9,6 +9,7 @@
 #include <linux/irqchip.h>
 #include <linux/irqdomain.h>
 #include <linux/module.h>
+#include <linux/scs.h>
 #include <linux/seq_file.h>
 #include <asm/sbi.h>
 #include <asm/smp.h>
@@ -34,6 +35,24 @@ EXPORT_SYMBOL_GPL(riscv_get_intc_hwnode);
 #ifdef CONFIG_IRQ_STACKS
 #include <asm/irq_stack.h>
 
+DECLARE_PER_CPU(ulong *, irq_shadow_call_stack_ptr);
+
+#ifdef CONFIG_SHADOW_CALL_STACK
+DEFINE_PER_CPU(ulong *, irq_shadow_call_stack_ptr);
+#endif
+
+static void init_irq_scs(void)
+{
+	int cpu;
+
+	if (!scs_is_enabled())
+		return;
+
+	for_each_possible_cpu(cpu)
+		per_cpu(irq_shadow_call_stack_ptr, cpu) =
+			scs_alloc(cpu_to_node(cpu));
+}
+
 DEFINE_PER_CPU(ulong *, irq_stack_ptr);
 
 #ifdef CONFIG_VMAP_STACK
@@ -61,40 +80,22 @@ static void init_irq_stacks(void)
 #endif /* CONFIG_VMAP_STACK */
 
 #ifdef CONFIG_HAVE_SOFTIRQ_ON_OWN_STACK
+static void ___do_softirq(struct pt_regs *regs)
+{
+	__do_softirq();
+}
+
 void do_softirq_own_stack(void)
 {
-#ifdef CONFIG_IRQ_STACKS
-	if (on_thread_stack()) {
-		ulong *sp = per_cpu(irq_stack_ptr, smp_processor_id())
-					+ IRQ_STACK_SIZE/sizeof(ulong);
-		__asm__ __volatile(
-		"addi	sp, sp, -"RISCV_SZPTR  "\n"
-		REG_S"  ra, (sp)		\n"
-		"addi	sp, sp, -"RISCV_SZPTR  "\n"
-		REG_S"  s0, (sp)		\n"
-		"addi	s0, sp, 2*"RISCV_SZPTR "\n"
-		"move	sp, %[sp]		\n"
-		"call	__do_softirq		\n"
-		"addi	sp, s0, -2*"RISCV_SZPTR"\n"
-		REG_L"  s0, (sp)		\n"
-		"addi	sp, sp, "RISCV_SZPTR   "\n"
-		REG_L"  ra, (sp)		\n"
-		"addi	sp, sp, "RISCV_SZPTR   "\n"
-		:
-		: [sp] "r" (sp)
-		: "a0", "a1", "a2", "a3", "a4", "a5", "a6", "a7",
-		  "t0", "t1", "t2", "t3", "t4", "t5", "t6",
-#ifndef CONFIG_FRAME_POINTER
-		  "s0",
-#endif
-		  "memory");
-	} else
-#endif
+	if (on_thread_stack())
+		call_on_irq_stack(NULL, ___do_softirq);
+	else
 		__do_softirq();
 }
 #endif /* CONFIG_HAVE_SOFTIRQ_ON_OWN_STACK */
 
 #else
+static void init_irq_scs(void) {}
 static void init_irq_stacks(void) {}
 #endif /* CONFIG_IRQ_STACKS */
 
@@ -106,6 +107,7 @@ int arch_show_interrupts(struct seq_file *p, int prec)
 
 void __init init_IRQ(void)
 {
+	init_irq_scs();
 	init_irq_stacks();
 	irqchip_init();
 	if (!handle_arch_irq)
diff --git a/arch/riscv/kernel/suspend_entry.S b/arch/riscv/kernel/suspend_entry.S
index f7960c7c5f9e..d5cf8b575777 100644
--- a/arch/riscv/kernel/suspend_entry.S
+++ b/arch/riscv/kernel/suspend_entry.S
@@ -61,10 +61,7 @@ END(__cpu_suspend_enter)
 
 SYM_TYPED_FUNC_START(__cpu_resume_enter)
 	/* Load the global pointer */
-	.option push
-	.option norelax
-		la gp, __global_pointer$
-	.option pop
+	load_global_pointer
 
 #ifdef CONFIG_MMU
 	/* Save A0 and A1 */
diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
index 19807c4d3805..cda6dcdb0376 100644
--- a/arch/riscv/kernel/traps.c
+++ b/arch/riscv/kernel/traps.c
@@ -352,34 +352,10 @@ static void noinstr handle_riscv_irq(struct pt_regs *regs)
 asmlinkage void noinstr do_irq(struct pt_regs *regs)
 {
 	irqentry_state_t state = irqentry_enter(regs);
-#ifdef CONFIG_IRQ_STACKS
-	if (on_thread_stack()) {
-		ulong *sp = per_cpu(irq_stack_ptr, smp_processor_id())
-					+ IRQ_STACK_SIZE/sizeof(ulong);
-		__asm__ __volatile(
-		"addi	sp, sp, -"RISCV_SZPTR  "\n"
-		REG_S"  ra, (sp)		\n"
-		"addi	sp, sp, -"RISCV_SZPTR  "\n"
-		REG_S"  s0, (sp)		\n"
-		"addi	s0, sp, 2*"RISCV_SZPTR "\n"
-		"move	sp, %[sp]		\n"
-		"move	a0, %[regs]		\n"
-		"call	handle_riscv_irq	\n"
-		"addi	sp, s0, -2*"RISCV_SZPTR"\n"
-		REG_L"  s0, (sp)		\n"
-		"addi	sp, sp, "RISCV_SZPTR   "\n"
-		REG_L"  ra, (sp)		\n"
-		"addi	sp, sp, "RISCV_SZPTR   "\n"
-		:
-		: [sp] "r" (sp), [regs] "r" (regs)
-		: "a0", "a1", "a2", "a3", "a4", "a5", "a6", "a7",
-		  "t0", "t1", "t2", "t3", "t4", "t5", "t6",
-#ifndef CONFIG_FRAME_POINTER
-		  "s0",
-#endif
-		  "memory");
-	} else
-#endif
+
+	if (IS_ENABLED(CONFIG_IRQ_STACKS) && on_thread_stack())
+		call_on_irq_stack(regs, handle_riscv_irq);
+	else
 		handle_riscv_irq(regs);
 
 	irqentry_exit(regs, state);
@@ -402,48 +378,14 @@ int is_valid_bugaddr(unsigned long pc)
 #endif /* CONFIG_GENERIC_BUG */
 
 #ifdef CONFIG_VMAP_STACK
-/*
- * Extra stack space that allows us to provide panic messages when the kernel
- * has overflowed its stack.
- */
-static DEFINE_PER_CPU(unsigned long [OVERFLOW_STACK_SIZE/sizeof(long)],
+DEFINE_PER_CPU(unsigned long [OVERFLOW_STACK_SIZE/sizeof(long)],
 		overflow_stack)__aligned(16);
-/*
- * A temporary stack for use by handle_kernel_stack_overflow.  This is used so
- * we can call into C code to get the per-hart overflow stack.  Usage of this
- * stack must be protected by spin_shadow_stack.
- */
-long shadow_stack[SHADOW_OVERFLOW_STACK_SIZE/sizeof(long)] __aligned(16);
-
-/*
- * A pseudo spinlock to protect the shadow stack from being used by multiple
- * harts concurrently.  This isn't a real spinlock because the lock side must
- * be taken without a valid stack and only a single register, it's only taken
- * while in the process of panicing anyway so the performance and error
- * checking a proper spinlock gives us doesn't matter.
- */
-unsigned long spin_shadow_stack;
-
-asmlinkage unsigned long get_overflow_stack(void)
-{
-	return (unsigned long)this_cpu_ptr(overflow_stack) +
-		OVERFLOW_STACK_SIZE;
-}
 
 asmlinkage void handle_bad_stack(struct pt_regs *regs)
 {
 	unsigned long tsk_stk = (unsigned long)current->stack;
 	unsigned long ovf_stk = (unsigned long)this_cpu_ptr(overflow_stack);
 
-	/*
-	 * We're done with the shadow stack by this point, as we're on the
-	 * overflow stack.  Tell any other concurrent overflowing harts that
-	 * they can proceed with panicing by releasing the pseudo-spinlock.
-	 *
-	 * This pairs with an amoswap.aq in handle_kernel_stack_overflow.
-	 */
-	smp_store_release(&spin_shadow_stack, 0);
-
 	console_verbose();
 
 	pr_emerg("Insufficient stack space to handle exception!\n");
diff --git a/arch/riscv/kernel/vdso/Makefile b/arch/riscv/kernel/vdso/Makefile
index 6b1dba11bf6d..48c362c0cb3d 100644
--- a/arch/riscv/kernel/vdso/Makefile
+++ b/arch/riscv/kernel/vdso/Makefile
@@ -36,7 +36,7 @@ CPPFLAGS_vdso.lds += -DHAS_VGETTIMEOFDAY
 endif
 
 # Disable -pg to prevent insert call site
-CFLAGS_REMOVE_vgettimeofday.o = $(CC_FLAGS_FTRACE)
+CFLAGS_REMOVE_vgettimeofday.o = $(CC_FLAGS_FTRACE) $(CC_FLAGS_SCS)
 
 # Disable profiling and instrumentation for VDSO code
 GCOV_PROFILE := n