[PATCH v2] riscv: fix race when vmap stack overflow

[Jisheng Zhang]

Currently, when detecting vmap stack overflow, riscv firstly switches
to the so called shadow stack, then use this shadow stack to call the
get_overflow_stack() to get the overflow stack. However, there's
a race here if two or more harts use the same shadow stack at the same
time.

To solve this race, we introduce spin_shadow_stack atomic var, which
will be swap between its own address and 0 in atomic way, when the
var is set, it means the shadow_stack is being used; when the var
is cleared, it means the shadow_stack isn't being used.

Fixes: 31da94c25aea ("riscv: add VMAP_STACK overflow detection")
Signed-off-by: Jisheng Zhang <jszhang@xxxxxxxxxx>
Suggested-by: Guo Ren <guoren@xxxxxxxxxx>
---
Since v1:
 - use smp_store_release directly
 - use unsigned int instead of atomic_t

 arch/riscv/kernel/entry.S | 4 ++++
 arch/riscv/kernel/traps.c | 4 ++++
 2 files changed, 8 insertions(+)

diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S
index b9eda3fcbd6d..7b924b16792b 100644
--- a/arch/riscv/kernel/entry.S
+++ b/arch/riscv/kernel/entry.S
@@ -404,6 +404,10 @@ handle_syscall_trace_exit:
 
 #ifdef CONFIG_VMAP_STACK
 handle_kernel_stack_overflow:
+1:	la sp, spin_shadow_stack
+	amoswap.w sp, sp, (sp)
+	bnez sp, 1b
+
 	la sp, shadow_stack
 	addi sp, sp, SHADOW_OVERFLOW_STACK_SIZE
 
diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
index f3e96d60a2ff..f1f57c1241b6 100644
--- a/arch/riscv/kernel/traps.c
+++ b/arch/riscv/kernel/traps.c
@@ -221,11 +221,15 @@ asmlinkage unsigned long get_overflow_stack(void)
 		OVERFLOW_STACK_SIZE;
 }
 
+unsigned int spin_shadow_stack;
+
 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);
 
+	smp_store_release(&spin_shadow_stack, 0);
+
 	console_verbose();
 
 	pr_emerg("Insufficient stack space to handle exception!\n");
-- 
2.37.2