net: in virtio_net_hdr only add VLAN_HLEN to csum_start if payload holds vlan

Tun, tap, virtio, packet and uml vector all use struct virtio_net_hdr
to communicate packet metadata to userspace.

For skbuffs with vlan, the first two return the packet as it may have
existed on the wire, inserting the VLAN tag in the user buffer.  Then
virtio_net_hdr.csum_start needs to be adjusted by VLAN_HLEN bytes.

Commit f09e2249 ("macvtap: restore vlan header on user read")
added this feature to macvtap. Commit 3ce9b20f ("macvtap: Fix
csum_start when VLAN tags are present") then fixed up csum_start.

Virtio, packet and uml do not insert the vlan header in the user
buffer.

When introducing virtio_net_hdr_from_skb to deduplicate filling in
the virtio_net_hdr, the variant from macvtap which adds VLAN_HLEN was
applied uniformly, breaking csum offset for packets with vlan on
virtio and packet.

Make insertion of VLAN_HLEN optional. Convert the callers to pass it
when needed.

Fixes: e858fae2 ("virtio_net: use common code for virtio_net_hdr and skb GSO conversion")
Fixes: 1276f24e ("packet: use common code for virtio_net_hdr and skb GSO conversion")
Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

arch/um/drivers/vector_transports.c

@@ -120,7 +120,8 @@ static int raw_form_header(uint8_t *header,
 		skb,
 		vheader,
 		virtio_legacy_is_little_endian(),
-		false
+		false,
+		0
 	);
 
 	return 0;

drivers/net/tap.c

@@ -774,13 +774,16 @@ static ssize_t tap_put_user(struct tap_queue *q,
 	int total;
 
 	if (q->flags & IFF_VNET_HDR) {
+		int vlan_hlen = skb_vlan_tag_present(skb) ? VLAN_HLEN : 0;
 		struct virtio_net_hdr vnet_hdr;
+
 		vnet_hdr_len = READ_ONCE(q->vnet_hdr_sz);
 		if (iov_iter_count(iter) < vnet_hdr_len)
 			return -EINVAL;
 
 		if (virtio_net_hdr_from_skb(skb, &vnet_hdr,
-					    tap_is_little_endian(q), true))
+					    tap_is_little_endian(q), true,
+					    vlan_hlen))
 			BUG();
 
 		if (copy_to_iter(&vnet_hdr, sizeof(vnet_hdr), iter) !=

drivers/net/tun.c

@@ -2089,7 +2089,8 @@ static ssize_t tun_put_user(struct tun_struct *tun,
 			return -EINVAL;
 
 		if (virtio_net_hdr_from_skb(skb, &gso,
-					    tun_is_little_endian(tun), true)) {
+					    tun_is_little_endian(tun), true,
+					    vlan_hlen)) {
 			struct skb_shared_info *sinfo = skb_shinfo(skb);
 			pr_err("unexpected GSO type: "
 			       "0x%x, gso_size %d, hdr_len %d\n",

drivers/net/virtio_net.c

@@ -1411,7 +1411,8 @@ static int xmit_skb(struct send_queue *sq, struct sk_buff *skb)
 		hdr = skb_vnet_hdr(skb);
 
 	if (virtio_net_hdr_from_skb(skb, &hdr->hdr,
-				    virtio_is_little_endian(vi->vdev), false))
+				    virtio_is_little_endian(vi->vdev), false,
+				    0))
 		BUG();
 
 	if (vi->mergeable_rx_bufs)

include/linux/virtio_net.h

@@ -58,7 +58,8 @@ static inline int virtio_net_hdr_to_skb(struct sk_buff *skb,
 static inline int virtio_net_hdr_from_skb(const struct sk_buff *skb,
 					  struct virtio_net_hdr *hdr,
 					  bool little_endian,
-					  bool has_data_valid)
+					  bool has_data_valid,
+					  int vlan_hlen)
 {
 	memset(hdr, 0, sizeof(*hdr));   /* no info leak */
 
@@ -83,12 +84,8 @@ static inline int virtio_net_hdr_from_skb(const struct sk_buff *skb,
 
 	if (skb->ip_summed == CHECKSUM_PARTIAL) {
 		hdr->flags = VIRTIO_NET_HDR_F_NEEDS_CSUM;
-		if (skb_vlan_tag_present(skb))
-			hdr->csum_start = __cpu_to_virtio16(little_endian,
-				skb_checksum_start_offset(skb) + VLAN_HLEN);
-		else
-			hdr->csum_start = __cpu_to_virtio16(little_endian,
-				skb_checksum_start_offset(skb));
+		hdr->csum_start = __cpu_to_virtio16(little_endian,
+			skb_checksum_start_offset(skb) + vlan_hlen);
 		hdr->csum_offset = __cpu_to_virtio16(little_endian,
 				skb->csum_offset);
 	} else if (has_data_valid &&

net/packet/af_packet.c

@@ -2005,7 +2005,7 @@ static int packet_rcv_vnet(struct msghdr *msg, const struct sk_buff *skb,
 		return -EINVAL;
 	*len -= sizeof(vnet_hdr);
 
-	if (virtio_net_hdr_from_skb(skb, &vnet_hdr, vio_le(), true))
+	if (virtio_net_hdr_from_skb(skb, &vnet_hdr, vio_le(), true, 0))
 		return -EINVAL;
 
 	return memcpy_to_msg(msg, (void *)&vnet_hdr, sizeof(vnet_hdr));
@@ -2272,7 +2272,7 @@ static int tpacket_rcv(struct sk_buff *skb, struct net_device *dev,
 	if (do_vnet) {
 		if (virtio_net_hdr_from_skb(skb, h.raw + macoff -
 					    sizeof(struct virtio_net_hdr),
-					    vio_le(), true)) {
+					    vio_le(), true, 0)) {
 			spin_lock(&sk->sk_receive_queue.lock);
 			goto drop_n_account;
 		}