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 <xietangxin@h-partners.com>
Closes: https://lore.kernel.org/netdev/99d678ae-c7b2-4b44-b534-b8320679deb3@h-partners.com/
Cc: <stable@vger.kernel.org> # 3.19: e16e960d55a4: ipvlan: inherit needed_headroom and needed_tailroom from phy_dev
Cc: <stable@vger.kernel.org> # 3.19: cef51860becd: macvlan: inherit needed_headroom and needed_tailroom from lowerdev
Cc: <stable@vger.kernel.org> # 3.19
Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260811085246.2267779-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

0 files changed, 0 insertions, 0 deletions