RISC-V: Fix a race condition during kernel stack overflow

[linux-2.6-microblaze.git] / arch / riscv / kernel / traps.c

diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
index f3e96d6..7abd8e4 100644 (file)
--- a/arch/riscv/kernel/traps.c
+++ b/arch/riscv/kernel/traps.c
@@ -221,11 +221,29 @@ asmlinkage unsigned long get_overflow_stack(void)
                OVERFLOW_STACK_SIZE;
 }
 
+/*
+ * 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 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");