ppp_push(struct ppp *ppp)
{
	struct list_head *list;
	struct channel *pch;
	struct sk_buff *skb = ppp->xmit_pending;

	if (!skb)
		return;

	list = &ppp->channels;
	if (list_empty(list)) {
		/* nowhere to send the packet, just drop it */
		ppp->xmit_pending = NULL;
		kfree_skb(skb);
		return;
	}

	if ((ppp->flags & SC_MULTILINK) == 0) {
		/* not doing multilink: send it down the first channel */
		list = list->next;
		pch = list_entry(list, struct channel, clist);

		spin_lock_bh(&pch->downl);
		if (pch->chan) {
			if (pch->chan->ops->start_xmit(pch->chan, skb))
				ppp->xmit_pending = NULL;
		} else {
			/* channel got unregistered */
			kfree_skb(skb);
			ppp->xmit_pending = NULL;
		}
		spin_unlock_bh(&pch->downl);
		return;
	}

#ifdef CONFIG_PPP_MULTILINK
	/* Multilink: fragment the packet over as many links
	   as can take the packet at the moment. */
	if (!ppp_mp_explode(ppp, skb))
		return;
#endif /* CONFIG_PPP_MULTILINK */

	ppp->xmit_pending = NULL;
	kfree_skb(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 Any operations using the original pointer are no longer valid because the memory belongs to the code that operates on the new pointer.