[PATCH net] vlan: fix skb_under_panic() and races when toggling HW VLAN offload

[Eric Dumazet <edumazet@google.com>]

Toggling hardware VLAN TX offload (NETIF_F_HW_VLAN_CTAG_TX or
NETIF_F_HW_VLAN_STAG_TX) on a lower device invokes vlan_transfer_features(),
which dynamically changes vlandev->hard_header_len.

This causes two issues:
1. Lockless TX paths (e.g. packet_snd in af_packet.c, ip6_finish_output2)
   read dev->hard_header_len without holding RTNL lock. Mutating
   hard_header_len dynamically under RTNL creates a data race where upper
   layers reserve insufficient headroom based on a stale hard_header_len,
   resulting in skb_under_panic when vlan_dev_hard_header() is called.
2. In addition, vlan_transfer_features() updated hard_header_len without
   updating header_ops, causing a mismatch between allocated headroom
   and header creation.

A VLAN interface may require software VLAN header insertion at any point
(e.g., if reorder_hdr is disabled or HW offload is unavailable). Always
setting hard_header_len = real_dev->hard_header_len + VLAN_HLEN and using
vlan_header_ops unconditionally ensures sufficient headroom is reserved
by all upper layers and avoids any data race on hard_header_len during ETHTOOL
feature changes.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Reported-by: Tangxin Xie <xietangxin@h-partners.com>
Closes: https://lore.kernel.org/netdev/99d678ae-c7b2-4b44-b534-b8320679deb3@h-partners.com/
Signed-off-by: Eric Dumazet <edumazet@google.com>
---
 net/8021q/vlan_dev.c | 34 +++-------------------------------
 1 file changed, 3 insertions(+), 31 deletions(-)

diff --git a/net/8021q/vlan_dev.c b/net/8021q/vlan_dev.c
index ec2569b3f8dac629027b4344bc89402decf026d1..acf7f52d1eb2191ec0387dc29e2e9eb34124aa9d 100644
--- a/net/8021q/vlan_dev.c
+++ b/net/8021q/vlan_dev.c
@@ -502,26 +502,6 @@ static const struct header_ops vlan_header_ops = {
 	.parse_protocol = vlan_parse_protocol,
 };
 
-static int vlan_passthru_hard_header(struct sk_buff *skb, struct net_device *dev,
-				     unsigned short type,
-				     const void *daddr, const void *saddr,
-				     unsigned int len)
-{
-	struct vlan_dev_priv *vlan = vlan_dev_priv(dev);
-	struct net_device *real_dev = vlan->real_dev;
-
-	if (saddr == NULL)
-		saddr = dev->dev_addr;
-
-	return dev_hard_header(skb, real_dev, type, daddr, saddr, len);
-}
-
-static const struct header_ops vlan_passthru_header_ops = {
-	.create	 = vlan_passthru_hard_header,
-	.parse	 = eth_header_parse,
-	.parse_protocol = vlan_parse_protocol,
-};
-
 static const struct device_type vlan_type = {
 	.name	= "vlan",
 };
@@ -581,13 +561,8 @@ static int vlan_dev_init(struct net_device *dev)
 #endif
 
 	dev->needed_headroom = real_dev->needed_headroom;
-	if (vlan_hw_offload_capable(real_dev->features, vlan->vlan_proto)) {
-		dev->header_ops      = &vlan_passthru_header_ops;
-		dev->hard_header_len = real_dev->hard_header_len;
-	} else {
-		dev->header_ops      = &vlan_header_ops;
-		dev->hard_header_len = real_dev->hard_header_len + VLAN_HLEN;
-	}
+	dev->header_ops      = &vlan_header_ops;
+	dev->hard_header_len = real_dev->hard_header_len + VLAN_HLEN;
 
 	dev->netdev_ops = &vlan_netdev_ops;
 
@@ -1029,10 +1004,7 @@ static void vlan_transfer_features(struct net_device *dev,
 
 	netif_inherit_tso_max(vlandev, dev);
 
-	if (vlan_hw_offload_capable(dev->features, vlan->vlan_proto))
-		vlandev->hard_header_len = dev->hard_header_len;
-	else
-		vlandev->hard_header_len = dev->hard_header_len + VLAN_HLEN;
+	vlandev->hard_header_len = dev->hard_header_len + VLAN_HLEN;
 
 #if IS_ENABLED(CONFIG_FCOE)
 	vlandev->fcoe_ddp_xid = dev->fcoe_ddp_xid;
-- 
2.55.0.229.g6434b31f56-goog