Merge branch 'vlan-fix'

Toshiaki Makita says:

====================
Fix vlan tag handling for vlan packets without ethernet headers

Eric Dumazet reported syzbot found a new bug which leads to underflow of
size argument of memmove(), causing crash[1]. This can be triggered by tun
devices.

The underflow happened because skb_vlan_untag() did not expect vlan packets
without ethernet headers, and tun can produce such packets.
I also checked vlan_insert_inner_tag() and found a similar bug.

This series fixes these problems.

[1] https://marc.info/?l=linux-netdev&m=152221753920510&w=2
====================
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/if_vlan.h

@@ -323,13 +323,24 @@ static inline int __vlan_insert_inner_tag(struct sk_buff *skb,
 	skb_push(skb, VLAN_HLEN);
 
 	/* Move the mac header sans proto to the beginning of the new header. */
+	if (likely(mac_len > ETH_TLEN))
 		memmove(skb->data, skb->data + VLAN_HLEN, mac_len - ETH_TLEN);
 	skb->mac_header -= VLAN_HLEN;
 
 	veth = (struct vlan_ethhdr *)(skb->data + mac_len - ETH_HLEN);
 
 	/* first, the ethernet type */
+	if (likely(mac_len >= ETH_TLEN)) {
+		/* h_vlan_encapsulated_proto should already be populated, and
+		 * skb->data has space for h_vlan_proto
+		 */
 		veth->h_vlan_proto = vlan_proto;
+	} else {
+		/* h_vlan_encapsulated_proto should not be populated, and
+		 * skb->data has no space for h_vlan_proto
+		 */
+		veth->h_vlan_encapsulated_proto = skb->protocol;
+	}
 
 	/* now, the TCI */
 	veth->h_vlan_TCI = htons(vlan_tci);

net/core/skbuff.c

@@ -5028,8 +5028,10 @@ static struct sk_buff *skb_reorder_vlan_header(struct sk_buff *skb)
 	}
 
 	mac_len = skb->data - skb_mac_header(skb);
+	if (likely(mac_len > VLAN_HLEN + ETH_TLEN)) {
 		memmove(skb_mac_header(skb) + VLAN_HLEN, skb_mac_header(skb),
 			mac_len - VLAN_HLEN - ETH_TLEN);
+	}
 	skb->mac_header += VLAN_HLEN;
 	return skb;
 }