static int l2tp_ip6_recv(struct sk_buff *skb)
{
	struct net *net = dev_net(skb->dev);
	struct sock *sk;
	u32 session_id;
	u32 tunnel_id;
	unsigned char *ptr, *optr;
	struct l2tp_session *session;
	struct l2tp_tunnel *tunnel = NULL;
	int length;

	if (!pskb_may_pull(skb, 4))
		goto discard;

	/* Point to L2TP header */
	optr = ptr = skb->data;
	session_id = ntohl(*((__be32 *) ptr));
	ptr += 4;

	/* RFC3931: L2TP/IP packets have the first 4 bytes containing
	 * the session_id. If it is 0, the packet is a L2TP control
	 * frame and the session_id value can be discarded.
	 */
	if (session_id == 0) {
		__skb_pull(skb, 4);
		goto pass_up;
	}

	/* Ok, this is a data packet. Lookup the session. */
	session = l2tp_session_find(net, NULL, session_id);
	if (session == NULL)
		goto discard;

	tunnel = session->tunnel;
	if (tunnel == NULL)
		goto discard;

	/* Trace packet contents, if enabled */
	if (tunnel->debug & L2TP_MSG_DATA) {
		length = min(32u, skb->len);
		if (!pskb_may_pull(skb, length))
			goto discard;

		/* Point to L2TP header */
		optr = ptr = skb->data;
		ptr += 4;
		pr_debug("%s: ip recv\n", tunnel->name);
		print_hex_dump_bytes("", DUMP_PREFIX_OFFSET, ptr, length);
	}

	l2tp_recv_common(session, skb, ptr, optr, 0, skb->len,
			 tunnel->recv_payload_hook);
	return 0;

pass_up:
	/* Get the tunnel_id from the L2TP header */
	if (!pskb_may_pull(skb, 12))
		goto discard;

	if ((skb->data[0] & 0xc0) != 0xc0)
		goto discard;

	tunnel_id = ntohl(*(__be32 *) &skb->data[4]);
	tunnel = l2tp_tunnel_find(net, tunnel_id);
	if (tunnel != NULL)
		sk = tunnel->sock;
	else {
		struct ipv6hdr *iph = ipv6_hdr(skb);

		read_lock_bh(&l2tp_ip6_lock);
		sk = __l2tp_ip6_bind_lookup(net, &iph->daddr,
					    0, tunnel_id);
		read_unlock_bh(&l2tp_ip6_lock);
	}

	if (sk == NULL)
		goto discard;

	sock_hold(sk);

	if (!xfrm6_policy_check(sk, XFRM_POLICY_IN, skb))
		goto discard_put;

	nf_reset(skb);

	return sk_receive_skb(sk, skb, 1);

discard_put:
	sock_put(sk);

discard:
	kfree_skb(skb);
	return 0;
}

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 Any operations using the original pointer are no longer valid because the memory belongs to the code that operates on the new pointer.