[PATCH net v3] net/packet: fix network header offset for non-VLAN raw packets on VLAN subinterfaces

AF_PACKET SOCK_RAW sets skb network_header to dev->hard_header_len in
packet_snd(). On VLAN subinterfaces with software tag insertion,
hard_header_len includes space for the VLAN tag (ETH_HLEN + VLAN_HLEN =
18) while min_header_len is the on-wire Ethernet header length
(ETH_HLEN = 14). A non-VLAN SOCK_RAW frame carries a standard 14-byte
Ethernet header, so its L3 header sits at min_header_len, not
hard_header_len.

packet_parse_headers() only corrects network_header for VLAN-tagged
frames. For non-VLAN frames it leaves network_header at hard_header_len,
so the IP header is found VLAN_HLEN bytes too late and
inet_gso_segment() fails with -EINVAL.

Observed on a virtio_net NIC (KVM guest) that advertises
NETIF_F_HW_VLAN_CTAG_FILTER but not NETIF_F_HW_VLAN_CTAG_TX, so VLAN
subinterfaces use software tag insertion (hard_header_len = 18). An
AF_PACKET SOCK_RAW socket bound to the VLAN subinterface with
PACKET_VNET_HDR enabled sends a large IPv4/TCP frame exceeding the path
MTU, with gso_type set in the virtio-net header. The user frame is a
plain [ethhdr][IP...] layout without a VLAN tag;
vlan_dev_hard_start_xmit() only attaches the tag metadata via
__vlan_hwaccel_put_tag(), and validate_xmit_vlan() on the physical
device later inserts the actual tag bytes via
__vlan_hwaccel_push_inside(). With network_header stuck at 18 while the
real IP header is at ETH_HLEN (14), inet_gso_segment() reads a
misaligned ip_hdr(skb) and returns -EINVAL.

For non-VLAN SOCK_RAW frames on VLAN subinterfaces, set network_header
to min_header_len so that the L3/L4 header positions match the actual
on-the-wire frame.

This fix is placed before skb_probe_transport_header() so that both the
transport header probe (which uses skb_network_offset() as nhoff) and
subsequent GSO see the right L3/L4 offsets. It complements
commit 01fdecc0480d ("net: packet: fix wrong transport_header when sending VLAN-tagged frame")
which only covers VLAN-tagged frames.

Fixes: dfed913e8b55 ("net/af_packet: add VLAN support for AF_PACKET SOCK_RAW GSO")
Signed-off-by: Junnan Zhang <redacted>
Signed-off-by: Shouxin Sun <redacted>
Signed-off-by: Junnan Zhang <redacted>
---
v3:
- Use is_vlan_dev() instead of the min_header_len < hard_header_len
  heuristic to detect the affected device; the heuristic may have false
  positives for other protocols, and the ARPHRD_ETHER check excluded
  the other hard_header_len cases anyway, so scope the fix back to
  VLAN subinterfaces.
- Rename is_vlan to is_vlan_packet to disambiguate from is_vlan_dev().
- Correct where the tag is inserted: vlan_dev_hard_start_xmit() only
  attaches the tag metadata via __vlan_hwaccel_put_tag(); the tag
  bytes are inserted later by validate_xmit_vlan() on the physical
  device.
- Reword the subject and commit message back to the VLAN-subinterface
  scope.

v2: https://lore.kernel.org/all/20260831072034.40044-1-zhangjn_dev@163.com/ (local)
v1: https://lore.kernel.org/all/20260821085722.24036-1-zhangjn_dev@163.com/#t (local)
---
 net/packet/af_packet.c | 22 ++++++++++++++++++++--
 1 file changed, 20 insertions(+), 2 deletions(-)

diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index 1168bd6b09cd..3bada38b3556 100644
--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c

@@ -1935,6 +1935,7 @@ static int packet_rcv_spkt(struct sk_buff *skb, struct net_device *dev,
 static void packet_parse_headers(struct sk_buff *skb, struct socket *sock)
 {
 	int depth;
+	bool is_vlan_packet = false;
 
 	/* On TX skb->data is the L2 header; anchor it for all socket types. */
 	skb_reset_mac_header(skb);

@@ -1943,11 +1944,28 @@ static void packet_parse_headers(struct sk_buff *skb, struct socket *sock)
 	    sock->type == SOCK_RAW)
 		skb->protocol = dev_parse_header_protocol(skb);
 
+	if (likely(skb->dev->type == ARPHRD_ETHER)) {
+		is_vlan_packet = eth_type_vlan(skb->protocol);
+
+		/* For non-VLAN SOCK_RAW frames on VLAN subinterfaces with
+		 * software tag insertion, hard_header_len includes space
+		 * for the VLAN tag while min_header_len is the on-wire
+		 * Ethernet header length. The user frame carries a
+		 * standard Ethernet header, so its L3 sits at
+		 * min_header_len, not hard_header_len. Move
+		 * network_header to the actual L2/L3 boundary so the
+		 * transport header probe below and subsequent GSO see
+		 * the right L3.
+		 */
+		if (sock->type == SOCK_RAW && !is_vlan_packet &&
+		    is_vlan_dev(skb->dev))
+			skb_set_network_header(skb, skb->dev->min_header_len);
+	}
+
 	skb_probe_transport_header(skb);
 
 	/* Move network header to the right position for VLAN tagged packets */
-	if (likely(skb->dev->type == ARPHRD_ETHER) &&
-	    eth_type_vlan(skb->protocol) &&
+	if (is_vlan_packet &&
 	    vlan_get_protocol_and_depth(skb, skb->protocol, &depth) != 0)
 		skb_set_network_header(skb, depth);
 }

-- 
2.43.0