ppp_mp_reconstruct(struct ppp *ppp)
{
	u32 seq = ppp->nextseq;
	u32 minseq = ppp->minseq;
	struct sk_buff_head *list = &ppp->mrq;
	struct sk_buff *p, *tmp;
	struct sk_buff *head, *tail;
	struct sk_buff *skb = NULL;
	int lost = 0, len = 0;

	if (ppp->mrru == 0)	/* do nothing until mrru is set */
		return NULL;
	head = list->next;
	tail = NULL;
	skb_queue_walk_safe(list, p, tmp) {
	again:
		if (seq_before(PPP_MP_CB(p)->sequence, seq)) {
			/* this can't happen, anyway ignore the skb */
			netdev_err(ppp->dev, "ppp_mp_reconstruct bad "
				   "seq %u < %u\n",
				   PPP_MP_CB(p)->sequence, seq);
			__skb_unlink(p, list);
			kfree_skb(p);
			continue;
		}
		if (PPP_MP_CB(p)->sequence != seq) {
			u32 oldseq;
			/* Fragment `seq' is missing.  If it is after
			   minseq, it might arrive later, so stop here. */
			if (seq_after(seq, minseq))
				break;
			/* Fragment `seq' is lost, keep going. */
			lost = 1;
			oldseq = seq;
			seq = seq_before(minseq, PPP_MP_CB(p)->sequence)?
				minseq + 1: PPP_MP_CB(p)->sequence;

			if (ppp->debug & 1)
				netdev_printk(KERN_DEBUG, ppp->dev,
					      "lost frag %u..%u\n",
					      oldseq, seq-1);

			goto again;
		}

		/*
		 * At this point we know that all the fragments from
		 * ppp->nextseq to seq are either present or lost.
		 * Also, there are no complete packets in the queue
		 * that have no missing fragments and end before this
		 * fragment.
		 */

		/* B bit set indicates this fragment starts a packet */
		if (PPP_MP_CB(p)->BEbits & B) {
			head = p;
			lost = 0;
			len = 0;
		}

		len += p->len;

		/* Got a complete packet yet? */
		if (lost == 0 && (PPP_MP_CB(p)->BEbits & E) &&
		    (PPP_MP_CB(head)->BEbits & B)) {
			if (len > ppp->mrru + 2) {
				++ppp->dev->stats.rx_length_errors;
				netdev_printk(KERN_DEBUG, ppp->dev,
					      "PPP: reconstructed packet"
					      " is too long (%d)\n", len);
			} else {
				tail = p;
				break;
			}
			ppp->nextseq = seq + 1;
		}

		/*
		 * If this is the ending fragment of a packet,
		 * and we haven't found a complete valid packet yet,
		 * we can discard up to and including this fragment.
		 */
		if (PPP_MP_CB(p)->BEbits & E) {
			struct sk_buff *tmp2;

			skb_queue_reverse_walk_from_safe(list, p, tmp2) {
				if (ppp->debug & 1)
					netdev_printk(KERN_DEBUG, ppp->dev,
						      "discarding frag %u\n",
						      PPP_MP_CB(p)->sequence);
				__skb_unlink(p, list);
				kfree_skb(p);
			}
			head = skb_peek(list);
			if (!head)
				break;
		}
		++seq;
	}

	/* If we have a complete packet, copy it all into one skb. */
	if (tail != NULL) {
		/* If we have discarded any fragments,
		   signal a receive error. */
		if (PPP_MP_CB(head)->sequence != ppp->nextseq) {
			skb_queue_walk_safe(list, p, tmp) {
				if (p == head)
					break;
				if (ppp->debug & 1)
					netdev_printk(KERN_DEBUG, ppp->dev,
						      "discarding frag %u\n",
						      PPP_MP_CB(p)->sequence);
				__skb_unlink(p, list);
				kfree_skb(p);
			}

			if (ppp->debug & 1)
				netdev_printk(KERN_DEBUG, ppp->dev,
					      "  missed pkts %u..%u\n",
					      ppp->nextseq,
					      PPP_MP_CB(head)->sequence-1);
			++ppp->dev->stats.rx_dropped;
			ppp_receive_error(ppp);
		}

		skb = head;
		if (head != tail) {
			struct sk_buff **fragpp = &skb_shinfo(skb)->frag_list;
			p = skb_queue_next(list, head);
			__skb_unlink(skb, list);
			skb_queue_walk_from_safe(list, p, tmp) {
				__skb_unlink(p, list);
				*fragpp = p;
				p->next = NULL;
				fragpp = &p->next;

				skb->len += p->len;
				skb->data_len += p->len;
				skb->truesize += p->truesize;

				if (p == tail)
					break;
			}
		} else {
			__skb_unlink(skb, list);
		}

		ppp->nextseq = PPP_MP_CB(tail)->sequence + 1;
	}

	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.