riscv: pmp: actually activate stack overflow protection during boot

Before this, stack protection would be effective only after switching to
the first thread.

Even before the first thread is created, the kernel init code uses the
IRQ stack to set things up. Let's make sure this is safeguarded as well.

This also fixes the incompatibility between CONFIG_RISCV_PMP and
CONFIG_RISCV_ALWAYS_SWITCH_THROUGH_ECALL, the later needing an exception
call to switch to the first thread and exception code assuming stack
guard is already set up in the PMP.

Signed-off-by: Nicolas Pitre <[email protected]>

arch/riscv/Kconfig

@@ -30,7 +30,6 @@ config RISCV_GP
 config RISCV_ALWAYS_SWITCH_THROUGH_ECALL
 	bool "Do not use mret outside a trap handler context"
 	depends on MULTITHREADING
-	depends on !RISCV_PMP
 	help
 	  Use mret instruction only when in a trap handler.
 	  This is for RISC-V implementations that require every mret to be

arch/riscv/core/pmp.c

@@ -348,8 +348,8 @@ static unsigned int global_pmp_end_index;
  */
 void z_riscv_pmp_init(void)
 {
-	unsigned long pmp_addr[4];
-	unsigned long pmp_cfg[1];
+	unsigned long pmp_addr[5];
+	unsigned long pmp_cfg[2];
 	unsigned int index = 0;
 
 	/* The read-only area is always there for every mode */
@@ -379,9 +379,27 @@ void z_riscv_pmp_init(void)
 		      (uintptr_t)z_interrupt_stacks[_current_cpu->id],
 		      Z_RISCV_STACK_GUARD_SIZE,
 		      pmp_addr, pmp_cfg, ARRAY_SIZE(pmp_addr));
-#endif
 
+	/*
+	 * This early, the kernel init code uses the IRQ stack and we want to
+	 * safeguard it as soon as possible. But we need a temporary default
+	 * "catch all" PMP entry for MPRV to work. Later on, this entry will
+	 * be set for each thread by z_riscv_pmp_stackguard_prepare().
+	 */
+	set_pmp_mprv_catchall(&index, pmp_addr, pmp_cfg, ARRAY_SIZE(pmp_addr));
+
+	 /* Write those entries to PMP regs. */
+	write_pmp_entries(0, index, true, pmp_addr, pmp_cfg, ARRAY_SIZE(pmp_addr));
+
+	/* Activate our non-locked PMP entries for m-mode */
+	csr_set(mstatus, MSTATUS_MPRV);
+
+	/* And forget about that last entry as we won't need it later */
+	index--;
+#else
+	 /* Write those entries to PMP regs. */
 	write_pmp_entries(0, index, true, pmp_addr, pmp_cfg, ARRAY_SIZE(pmp_addr));
+#endif
 
 #ifdef CONFIG_SMP
 #ifdef CONFIG_PMP_STACK_GUARD
@@ -401,6 +419,7 @@ void z_riscv_pmp_init(void)
 	}
 #endif
 
+	__ASSERT(index <= PMPCFG_STRIDE, "provision for one global word only");
 	global_pmp_cfg[0] = pmp_cfg[0];
 	global_pmp_last_addr = pmp_addr[index - 1];
 	global_pmp_end_index = index;