xtensa: initialize privileged stack during thread init

This adds the bits to initialize the privileged stack for
each thread during thread initialization. This prevents
information leaking if the thread stack is reused, and
also aids in calculating stack space usage during system
calls.

Signed-off-by: Daniel Leung <[email protected]>

Author: dcpleung

arch/xtensa/core/thread.c

@@ -72,6 +72,18 @@ static void *init_stack(struct k_thread *thread, int *stack_top,
 	frame->bsa.ps = PS_WOE | PS_UM | PS_CALLINC(1);
 #ifdef CONFIG_USERSPACE
 	if ((thread->base.user_options & K_USER) == K_USER) {
+#ifdef CONFIG_INIT_STACKS
+		/* setup_thread_stack() does not initialize the architecture specific
+		 * privileged stack. So we need to do it manually here as this function
+		 * is called by arch_new_thread() via z_setup_new_thread() after
+		 * setup_thread_stack() but before thread starts running.
+		 *
+		 * Note that only user threads have privileged stacks and kernel
+		 * only threads do not.
+		 */
+		(void)memset(&header->privilege_stack[0], 0xaa, sizeof(header->privilege_stack));
+#endif
+
 		frame->bsa.pc = (uintptr_t)arch_user_mode_enter;
 	} else {
 		frame->bsa.pc = (uintptr_t)z_thread_entry;