Message-ID: <2025072811-CVE-2025-38470-a4d4@gregkh>
Date: Mon, 28 Jul 2025 13:22:13 +0200
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: linux-cve-announce@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...nel.org>
Subject: CVE-2025-38470: net: vlan: fix VLAN 0 refcount imbalance of toggling filtering during runtime

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

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

In the Linux kernel, the following vulnerability has been resolved:

net: vlan: fix VLAN 0 refcount imbalance of toggling filtering during runtime

Assuming the "rx-vlan-filter" feature is enabled on a net device, the
8021q module will automatically add or remove VLAN 0 when the net device
is put administratively up or down, respectively. There are a couple of
problems with the above scheme.

The first problem is a memory leak that can happen if the "rx-vlan-filter"
feature is disabled while the device is running:

 # ip link add bond1 up type bond mode 0
 # ethtool -K bond1 rx-vlan-filter off
 # ip link del dev bond1

When the device is put administratively down the "rx-vlan-filter"
feature is disabled, so the 8021q module will not remove VLAN 0 and the
memory will be leaked [1].

Another problem that can happen is that the kernel can automatically
delete VLAN 0 when the device is put administratively down despite not
adding it when the device was put administratively up since during that
time the "rx-vlan-filter" feature was disabled. null-ptr-unref or
bug_on[2] will be triggered by unregister_vlan_dev() for refcount
imbalance if toggling filtering during runtime:

$ ip link add bond0 type bond mode 0
$ ip link add link bond0 name vlan0 type vlan id 0 protocol 802.1q
$ ethtool -K bond0 rx-vlan-filter off
$ ifconfig bond0 up
$ ethtool -K bond0 rx-vlan-filter on
$ ifconfig bond0 down
$ ip link del vlan0

Root cause is as below:
step1: add vlan0 for real_dev, such as bond, team.
register_vlan_dev
    vlan_vid_add(real_dev,htons(ETH_P_8021Q),0) //refcnt=1
step2: disable vlan filter feature and enable real_dev
step3: change filter from 0 to 1
vlan_device_event
    vlan_filter_push_vids
        ndo_vlan_rx_add_vid //No refcnt added to real_dev vlan0
step4: real_dev down
vlan_device_event
    vlan_vid_del(dev, htons(ETH_P_8021Q), 0); //refcnt=0
        vlan_info_rcu_free //free vlan0
step5: delete vlan0
unregister_vlan_dev
    BUG_ON(!vlan_info); //vlan_info is null

Fix both problems by noting in the VLAN info whether VLAN 0 was
automatically added upon NETDEV_UP and based on that decide whether it
should be deleted upon NETDEV_DOWN, regardless of the state of the
"rx-vlan-filter" feature.

[1]
unreferenced object 0xffff8880068e3100 (size 256):
  comm "ip", pid 384, jiffies 4296130254
  hex dump (first 32 bytes):
    00 20 30 0d 80 88 ff ff 00 00 00 00 00 00 00 00  . 0.............
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
  backtrace (crc 81ce31fa):
    __kmalloc_cache_noprof+0x2b5/0x340
    vlan_vid_add+0x434/0x940
    vlan_device_event.cold+0x75/0xa8
    notifier_call_chain+0xca/0x150
    __dev_notify_flags+0xe3/0x250
    rtnl_configure_link+0x193/0x260
    rtnl_newlink_create+0x383/0x8e0
    __rtnl_newlink+0x22c/0xa40
    rtnl_newlink+0x627/0xb00
    rtnetlink_rcv_msg+0x6fb/0xb70
    netlink_rcv_skb+0x11f/0x350
    netlink_unicast+0x426/0x710
    netlink_sendmsg+0x75a/0xc20
    __sock_sendmsg+0xc1/0x150
    ____sys_sendmsg+0x5aa/0x7b0
    ___sys_sendmsg+0xfc/0x180

[2]
kernel BUG at net/8021q/vlan.c:99!
Oops: invalid opcode: 0000 [#1] SMP KASAN PTI
CPU: 0 UID: 0 PID: 382 Comm: ip Not tainted 6.16.0-rc3 #61 PREEMPT(voluntary)
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996),
BIOS rel-1.13.0-0-gf21b5a4aeb02-prebuilt.qemu.org 04/01/2014
RIP: 0010:unregister_vlan_dev (net/8021q/vlan.c:99 (discriminator 1))
RSP: 0018:ffff88810badf310 EFLAGS: 00010246
RAX: 0000000000000000 RBX: ffff88810da84000 RCX: ffffffffb47ceb9a
RDX: dffffc0000000000 RSI: 0000000000000008 RDI: ffff88810e8b43c8
RBP: 0000000000000000 R08: 0000000000000000 R09: fffffbfff6cefe80
R10: ffffffffb677f407 R11: ffff88810badf3c0 R12: ffff88810e8b4000
R13: 0000000000000000 R14: ffff88810642a5c0 R15: 000000000000017e
FS:  00007f1ff68c20c0(0000) GS:ffff888163a24000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f1ff5dad240 CR3: 0000000107e56000 CR4: 00000000000006f0
Call Trace:
 <TASK>
rtnl_dellink (net/core/rtnetlink.c:3511 net/core/rtnetlink.c:3553)
rtnetlink_rcv_msg (net/core/rtnetlink.c:6945)
netlink_rcv_skb (net/netlink/af_netlink.c:2535)
netlink_unicast (net/netlink/af_netlink.c:1314 net/netlink/af_netlink.c:1339)
netlink_sendmsg (net/netlink/af_netlink.c:1883)
____sys_sendmsg (net/socket.c:712 net/socket.c:727 net/socket.c:2566)
___sys_sendmsg (net/socket.c:2622)
__sys_sendmsg (net/socket.c:2652)
do_syscall_64 (arch/x86/entry/syscall_64.c:63 arch/x86/entry/syscall_64.c:94)

The Linux kernel CVE team has assigned CVE-2025-38470 to this issue.


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

	Issue introduced in 2.6.36 with commit ad1afb00393915a51c21b1ae8704562bf036855f and fixed in 6.1.147 with commit d43ef15bf4856c8c4c6c3572922331a5f06deb77
	Issue introduced in 2.6.36 with commit ad1afb00393915a51c21b1ae8704562bf036855f and fixed in 6.6.100 with commit bb515c41306454937464da055609b5fb0a27821b
	Issue introduced in 2.6.36 with commit ad1afb00393915a51c21b1ae8704562bf036855f and fixed in 6.12.40 with commit 8984bcbd1edf5bee5be06ad771d157333b790c33
	Issue introduced in 2.6.36 with commit ad1afb00393915a51c21b1ae8704562bf036855f and fixed in 6.15.8 with commit 93715aa2d80e6c5cea1bb486321fc4585076928b
	Issue introduced in 2.6.36 with commit ad1afb00393915a51c21b1ae8704562bf036855f and fixed in 6.16 with commit 579d4f9ca9a9a605184a9b162355f6ba131f678d

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-2025-38470
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.c
	net/8021q/vlan.h


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/d43ef15bf4856c8c4c6c3572922331a5f06deb77
	https://git.kernel.org/stable/c/bb515c41306454937464da055609b5fb0a27821b
	https://git.kernel.org/stable/c/8984bcbd1edf5bee5be06ad771d157333b790c33
	https://git.kernel.org/stable/c/93715aa2d80e6c5cea1bb486321fc4585076928b
	https://git.kernel.org/stable/c/579d4f9ca9a9a605184a9b162355f6ba131f678d

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