struct sk_buff *__alloc_skb_head(gfp_t gfp_mask, int node)
{
	struct sk_buff *skb;

	/* Get the HEAD */
	skb = kmem_cache_alloc_node(skbuff_head_cache,
				    gfp_mask & ~__GFP_DMA, node);
	if (!skb)
		goto out;

	/*
	 * Only clear those fields we need to clear, not those that we will
	 * actually initialise below. Hence, don't put any more fields after
	 * the tail pointer in struct sk_buff!
	 */
	memset(skb, 0, offsetof(struct sk_buff, tail));
	skb->head = NULL;
	skb->truesize = sizeof(struct sk_buff);
	atomic_set(&skb->users, 1);

	skb->mac_header = (typeof(skb->mac_header))~0U;
out:
	return skb;
}

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 Any operations using the original pointer are no longer valid because the memory belongs to the code that operates on the new pointer.