[PATCH net v2] net/packet: fix network header offset for non-VLAN raw packets

[Junnan Zhang]

AF_PACKET SOCK_RAW sets skb network_header to dev->hard_header_len in
packet_snd(). For Ethernet devices whose hard_header_len exceeds the
on-wire L2 header length (min_header_len = ETH_HLEN) -- e.g.
software-offload VLAN subinterfaces, where hard_header_len = ETH_HLEN +
VLAN_HLEN = 18 -- a non-VLAN SOCK_RAW frame still 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 (hard_header_len - min_header_len) 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; the VLAN subdevice
inserts the 802.1Q tag in vlan_dev_hard_start_xmit(). 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.

Set network_header to min_header_len for non-VLAN SOCK_RAW frames on
Ethernet devices whose hard_header_len exceeds min_header_len, so 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 <zhangjn11@xxxxxxxxxxxxxxx>
Signed-off-by: Shouxin Sun <sunshx@xxxxxxxxxxxxxxx>
Signed-off-by: Junnan Zhang <zhangjn_dev@xxxxxxx>
---
v2:
- Drop "on VLAN subinterfaces" from the subject and reword the scope to
  Ethernet devices whose hard_header_len exceeds the on-wire L2 header
  length (min_header_len). The min_header_len < hard_header_len test is
  not VLAN-specific; it also covers Ethernet drivers that reserve extra
  hard_header_len space for driver-internal wrapping.
- Restructure packet_parse_headers() to test dev->type once; replace
  has_vlan, which mixed the device and packet tests, with the
  packet-only is_vlan.
- Describe the header offset error generically as (hard_header_len -
  min_header_len) bytes instead of hard-coding the 4-byte VLAN case.
- Document the reproducer: virtio_net advertising
  NETIF_F_HW_VLAN_CTAG_FILTER but not NETIF_F_HW_VLAN_CTAG_TX, with
  PACKET_VNET_HDR and GSO triggering the -EINVAL from
  inet_gso_segment().

v1: https://lore.kernel.org/all/20260821085722.24036-1-zhangjn_dev@xxxxxxx/#t
---
 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..be5bf9db7ea1 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 = 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 = eth_type_vlan(skb->protocol);
+
+		/* For non-VLAN SOCK_RAW frames on Ethernet devices whose
+		 * hard_header_len exceeds the on-wire L2 header length
+		 * (min_header_len) -- e.g. software-offload VLAN subinterfaces,
+		 * or Ethernet drivers that reserve extra space in
+		 * hard_header_len for driver-internal wrapping -- the SOCK_RAW
+		 * send paths leave network_header at hard_header_len, while the
+		 * user frame's L3 sits at min_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 (!is_vlan && sock->type == SOCK_RAW &&
+		    skb->dev->min_header_len < skb->dev->hard_header_len)
+			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 &&
 	    vlan_get_protocol_and_depth(skb, skb->protocol, &depth) != 0)
 		skb_set_network_header(skb, depth);
 }
-- 
2.43.0