From 447cbe95ebb95392b5d8f6a01c0556826919ce23 Mon Sep 17 00:00:00 2001 From: Eric Dumazet Date: Tue, 11 Aug 2026 08:52:46 +0000 Subject: [PATCH] vlan: fix skb_under_panic and races when toggling HW VLAN offload 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 changed 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. Always setting dev->hard_header_len = real_dev->hard_header_len and dev->needed_headroom = real_dev->needed_headroom + VLAN_HLEN unconditionally ensures: - dev->hard_header_len remains 100% static and immutable at real_dev->hard_header_len, eliminating all dynamic runtime updates and data races on hard_header_len. - Upper layers allocating skbs via LL_RESERVED_SPACE() will always reserve sufficient headroom for software VLAN tag insertion (real_dev->hard_header_len + real_dev->needed_headroom + VLAN_HLEN). - vlandev inherits real_dev->needed_tailroom so underlying trailer/padding/ICV requirements are honored. - AF_PACKET SOCK_RAW network header offsets remain correctly aligned at real_dev->hard_header_len. - vlan_header_ops is used unconditionally. Note to stable teams: Make sure to backport these commits: e16e960d55a4 ("ipvlan: inherit needed_headroom and needed_tailroom from phy_dev") cef51860becd ("macvlan: inherit needed_headroom and needed_tailroom from lowerdev") Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2") Reported-by: Tangxin Xie Closes: https://lore.kernel.org/netdev/99d678ae-c7b2-4b44-b534-b8320679deb3@h-partners.com/ Cc: # 3.19: e16e960d55a4: ipvlan: inherit needed_headroom and needed_tailroom from phy_dev Cc: # 3.19: cef51860becd: macvlan: inherit needed_headroom and needed_tailroom from lowerdev Cc: # 3.19 Signed-off-by: Eric Dumazet Link: https://patch.msgid.link/20260811085246.2267779-1-edumazet@google.com Signed-off-by: Jakub Kicinski --- net/8021q/vlan_dev.c | 39 +++++++-------------------------------- 1 file changed, 7 insertions(+), 32 deletions(-) diff --git a/net/8021q/vlan_dev.c b/net/8021q/vlan_dev.c index ec2569b3f8da..2859cbac3f26 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", }; @@ -580,14 +560,10 @@ static int vlan_dev_init(struct net_device *dev) dev->fcoe_ddp_xid = real_dev->fcoe_ddp_xid; #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->needed_headroom = real_dev->needed_headroom + VLAN_HLEN; + dev->needed_tailroom = real_dev->needed_tailroom; + dev->header_ops = &vlan_header_ops; + dev->hard_header_len = real_dev->hard_header_len; dev->netdev_ops = &vlan_netdev_ops; @@ -1029,10 +1005,9 @@ 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->needed_headroom = dev->needed_headroom + VLAN_HLEN; + vlandev->needed_tailroom = dev->needed_tailroom; + vlandev->hard_header_len = dev->hard_header_len; #if IS_ENABLED(CONFIG_FCOE) vlandev->fcoe_ddp_xid = dev->fcoe_ddp_xid;