Re: [BUG] vlan: skb_under_panic when toggling NETIF_F_HW_VLAN_CTAG_TX on lower device

[Eric Dumazet]

On Thu, Jul 16, 2026 at 2:53 PM Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
>
> On Thu, Jul 16, 2026 at 2:20 PM xietangxin <xietangxin@xxxxxxxxxxxxxx> wrote:
> >
> > [BUG] vlan: skb_under_panic when toggling NETIF_F_HW_VLAN_CTAG_TX on lower device
> >
> > Hi all,
> >
> > We encountered a skb_under_panic triggered by toggling
> > NETIF_F_HW_VLAN_CTAG_TX on the lower device while a VLAN device is
> > up and sending traffic.
> >
> > Call trace
> > ==========
> >
> >  skbuff: skb_under_panic: text:ffffc0d2900283d8 len:74 put:14
> >   head:ffff334820249c00 data:ffff334820249bfe tail:0x48 end:0xc0 dev:vlan4
> >  ------------[ cut here ]------------
> >  kernel BUG at net/core/skbuff.c:116!
> >  Internal error: Oops - BUG: 00000000f2000800 [#1] SMP
> >  Call trace:
> >   skb_panic+0xcc/0xd0
> >   __skb_checksum+0x0/0x480
> >   eth_header+0x48/0x1a0
> >   vlan_dev_hard_header+0xd0/0x284
> >   neigh_connected_output+0x16c/0x20c
> >   ip6_finish_output2+0x4b4/0xd74
> >   __ip6_finish_output.part.0+0x1ac/0x3b0
> >   ip6_finish_output+0x160/0x200
> >   ip6_output+0x13c/0x294
> >   ndisc_send_skb+0x41c/0x6f0
> >   ndisc_send_rs+0xac/0x3b0
> >   addrconf_rs_timer+0x42c/0x660
> >   call_timer_fn+0x54/0x290
> >   expire_timers+0x26c/0x420
> >
> > Reproducer
> > ==========
> >
> >  # Create veth pair (NETIF_F_HW_VLAN_CTAG_TX is ON by default)
> >  ip link add veth0 type veth peer name veth1
> >  ip link set veth0 up
> >  ip link set veth1 up
> >
> >  # Turn off HW VLAN TX offload on lower device
> >  ethtool -K veth0 tx-vlan-hw-insert off
> >
> >  # Create VLAN device on veth0
> >  # At this point: header_ops = &vlan_header_ops, hard_header_len = 18
> >  ip link add link veth0 name veth0.10 type vlan id 10 reorder_hdr off
> >  ip addr add 192.168.10.1/24 dev veth0.10
> >  ip link set veth0.10 up
> >
> >  # Turn HW VLAN TX offload back ON on lower device
> >  # This triggers NETDEV_FEAT_CHANGE -> vlan_transfer_features()
> >  # hard_header_len changes from 18 to 14, but header_ops is NOT updated
> >  ethtool -K veth0 tx-vlan-hw-insert on
> >
> >  # When a packet is sent through veth0.10
> >  # - skb is allocated based on hard_header_len=14 -> ~16 bytes
> >  # - vlan_dev_hard_header() pushes VLAN_HLEN(4) + ETH_HLEN(14) = 18 bytes
> >  # - skb_under_panic!
> >
> >
> > Any feedback or guidance would be greatly appreciated.
>
> This rings a bell, I thought we already fixed this issue long ago :/
>
> I would suggest we always add VLAN_HLEN even if not (yet) needed.
>
> diff --git a/net/8021q/vlan.c b/net/8021q/vlan.c
> index 2b74ed56eb166d52c3351768d9dfedc7b2c8ec2d..d7f90b3b2bb3aadb2c466891720a7f407d0bc34b
> 100644
> --- a/net/8021q/vlan.c
> +++ b/net/8021q/vlan.c
> @@ -323,10 +323,7 @@ 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;
>
> A kind-of-similar change is needed in net/8021q/vlan_dev.c

The second part would be a nice cleanup, unless I am missing something.

diff --git a/net/8021q/vlan_dev.c b/net/8021q/vlan_dev.c
index 7aa3af8b10ead0384bbb2e767883b63a37be5fa2..dfd907e2ede2dcb0b6120e1fa5b6bb4d30bfb28a
100644
--- a/net/8021q/vlan_dev.c
+++ b/net/8021q/vlan_dev.c
@@ -496,26 +496,6 @@ 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",
 };
@@ -575,13 +555,8 @@ 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;