CVE-2026-80925: vlan: fix skb_under_panic and races when toggling HW VLAN offload

CVE-2026-80925: vlan: fix skb_under_panic and races when toggling HW VLAN offload

[Greg Kroah-Hartman]

From: Greg Kroah-Hartman <gregkh@kernel.org>

Description
===========

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")

The Linux kernel CVE team has assigned CVE-2026-80925 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 2.6.12 with commit 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 and fixed in 7.2.3 with commit a29f3b884ba50217e6f50414f568073221e2bb07
	Issue introduced in 2.6.12 with commit 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 and fixed in 7.3-rc1 with commit 447cbe95ebb95392b5d8f6a01c0556826919ce23

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-80925
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	net/8021q/vlan_dev.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/a29f3b884ba50217e6f50414f568073221e2bb07
	https://git.kernel.org/stable/c/447cbe95ebb95392b5d8f6a01c0556826919ce23