int skb_vlan_push(struct sk_buff *skb, __be16 vlan_proto, u16 vlan_tci)
{
	if (skb_vlan_tag_present(skb)) {
		int offset = skb->data - skb_mac_header(skb);
		int err;

		if (WARN_ONCE(offset,
			      "skb_vlan_push got skb with skb->data not at mac header (offset %d)\n",
			      offset)) {
			return -EINVAL;
		}

		err = __vlan_insert_tag(skb, skb->vlan_proto,
					skb_vlan_tag_get(skb));
		if (err)
			return err;

		skb->protocol = skb->vlan_proto;
		skb->mac_len += VLAN_HLEN;

		skb_postpush_rcsum(skb, skb->data + (2 * ETH_ALEN), VLAN_HLEN);
	}
	__vlan_hwaccel_put_tag(skb, vlan_proto, vlan_tci);
	return 0;
}

The product reads data past the end, or before the beginning, of the intended buffer.