static unsigned long *alloc_thread_stack_node(struct task_struct *tsk, int node)
{
#ifdef CONFIG_VMAP_STACK
	void *stack;
	int i;

	for (i = 0; i < NR_CACHED_STACKS; i++) {
		struct vm_struct *s;

		s = this_cpu_xchg(cached_stacks[i], NULL);

		if (!s)
			continue;

		tsk->stack_vm_area = s;
		return s->addr;
	}

	stack = __vmalloc_node_range(THREAD_SIZE, THREAD_SIZE,
				     VMALLOC_START, VMALLOC_END,
				     THREADINFO_GFP,
				     PAGE_KERNEL,
				     0, node, __builtin_return_address(0));

	/*
	 * We can't call find_vm_area() in interrupt context, and
	 * free_thread_stack() can be called in interrupt context,
	 * so cache the vm_struct.
	 */
	if (stack)
		tsk->stack_vm_area = find_vm_area(stack);
	return stack;
#else
	struct page *page = alloc_pages_node(node, THREADINFO_GFP,
					     THREAD_SIZE_ORDER);

	return page ? page_address(page) : NULL;
#endif
}

The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory belongs to the code that operates on the new pointer.