arch: riscv: remove PMP stack guard for stack overflow handler

When RISCV_ALWAYS_SWITCH_THROUGH_ECALL is enabled, do_swap() enables PMP
checking in is_kernel_syscall.
If the PMP stack guard is triggered and do_swap() is called from the
fault handler, a PMP error occurs because the stack usage violates the
previous PMP setting.

Remove the stack guard setting during a stack overflow handler to allow
enabling PMP checking safely in fault handler.

Signed-off-by: Jimmy Zheng <[email protected]>

Author: jimmyzhe

arch/riscv/core/fatal.c

@@ -226,6 +226,13 @@ void _Fault(struct arch_esf *esf)
 	unsigned int reason = K_ERR_CPU_EXCEPTION;
 
 	if (bad_stack_pointer(esf)) {
+#ifdef CONFIG_PMP_STACK_GUARD
+		/*
+		 * Remove the thread's PMP setting to prevent triggering a stack
+		 * overflow error again due to the previous configuration.
+		 */
+		z_riscv_pmp_stackguard_disable();
+#endif /* CONFIG_PMP_STACK_GUARD */
 		reason = K_ERR_STACK_CHK_FAIL;
 	}
 

arch/riscv/core/pmp.c

@@ -511,6 +511,37 @@ void z_riscv_pmp_stackguard_enable(struct k_thread *thread)
 	csr_set(mstatus, MSTATUS_MPRV);
 }
 
+/**
+ * @brief Remove PMP stackguard content to actual PMP registers
+ */
+void z_riscv_pmp_stackguard_disable(void)
+{
+
+	unsigned long pmp_addr[PMP_M_MODE_SLOTS];
+	unsigned long pmp_cfg[PMP_M_MODE_SLOTS / sizeof(unsigned long)];
+	unsigned int index = global_pmp_end_index;
+
+	/* Retrieve the pmpaddr value matching the last global PMP slot. */
+	pmp_addr[global_pmp_end_index - 1] = global_pmp_last_addr;
+
+	/* Disable (non-locked) PMP entries for m-mode while we update them. */
+	csr_clear(mstatus, MSTATUS_MPRV);
+
+	/*
+	 * Set a temporary default "catch all" PMP entry for MPRV to work,
+	 * except for the global locked entries.
+	 */
+	set_pmp_mprv_catchall(&index, pmp_addr, pmp_cfg, ARRAY_SIZE(pmp_addr));
+
+	/* Write "catch all" entry and clear unlocked entries to PMP regs. */
+	write_pmp_entries(global_pmp_end_index, index,
+			  true, pmp_addr, pmp_cfg, ARRAY_SIZE(pmp_addr));
+
+	if (PMP_DEBUG_DUMP) {
+		dump_pmp_regs("catch all register dump");
+	}
+}
+
 #endif /* CONFIG_PMP_STACK_GUARD */
 
 #ifdef CONFIG_USERSPACE

arch/riscv/include/pmp.h

@@ -10,6 +10,7 @@
 void z_riscv_pmp_init(void);
 void z_riscv_pmp_stackguard_prepare(struct k_thread *thread);
 void z_riscv_pmp_stackguard_enable(struct k_thread *thread);
+void z_riscv_pmp_stackguard_disable(void);
 void z_riscv_pmp_usermode_init(struct k_thread *thread);
 void z_riscv_pmp_usermode_prepare(struct k_thread *thread);
 void z_riscv_pmp_usermode_enable(struct k_thread *thread);