net: Fix potential wrong skb->protocol in skb_vlan_untag()

[ Upstream commit 55eff0e ]

We may access the two bytes after vlan_hdr in vlan_set_encap_proto(). So
we should pull VLAN_HLEN + sizeof(unsigned short) in skb_vlan_untag() or
we may access the wrong data.

Fixes: 0d5501c ("net: Always untag vlan-tagged traffic on input.")
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

net/core/skbuff.c

@@ -5317,8 +5317,8 @@ struct sk_buff *skb_vlan_untag(struct sk_buff *skb)
 	skb = skb_share_check(skb, GFP_ATOMIC);
 	if (unlikely(!skb))
 		goto err_free;
-
-	if (unlikely(!pskb_may_pull(skb, VLAN_HLEN)))
+	/* We may access the two bytes after vlan_hdr in vlan_set_encap_proto(). */
+	if (unlikely(!pskb_may_pull(skb, VLAN_HLEN + sizeof(unsigned short))))
 		goto err_free;
 
 	vhdr = (struct vlan_hdr *)skb->data;