published September 9, 2026
In the Linux kernel, the following vulnerability has been resolved:
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")