x86: 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 <daniel.leung@intel.com>

arch/x86/core/userspace.c

@@ -93,6 +93,28 @@ void *z_x86_userspace_prepare_thread(struct k_thread *thread)
 
 	if ((thread->base.user_options & K_USER) != 0U) {
 		initial_entry = arch_user_mode_enter;
+
+#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.
+		 *
+		 * Also note that this needs to be done before calling
+		 * z_x86_userspace_enter() where it clears the user stack.
+		 * That function requires using the privileged stack for
+		 * code execution so we cannot clear that at the same time.
+		 */
+		struct z_x86_thread_stack_header *hdr_stack_obj =
+			(struct z_x86_thread_stack_header *)thread->stack_obj;
+
+		(void)memset(&hdr_stack_obj->privilege_stack[0], 0xaa,
+			     sizeof(hdr_stack_obj->privilege_stack));
+#endif
+
 	} else {
 		initial_entry = z_thread_entry;
 	}