CVE-2025-38470 in Linux
Summary
by MITRE • 07/28/2025
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 ---truncated---
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Analysis
by VulDB Data Team • 01/25/2026
The vulnerability CVE-2025-38470 affects the Linux kernel's implementation of VLAN filtering within the 8021q module, specifically concerning the handling of VLAN 0 during runtime toggling of the rx-vlan-filter feature. This issue arises when a network device such as a bond interface is administratively brought up or down while the rx-vlan-filter functionality is enabled or disabled. The root cause lies in improper reference counting mechanisms for VLAN 0, which leads to either memory leaks or kernel panics due to null pointer dereferences.
When a network device is brought up with rx-vlan-filter enabled, the 8021q module automatically adds VLAN 0 to the device. However, if the rx-vlan-filter feature is subsequently disabled while the device is running, the module fails to remove VLAN 0 during device shutdown, resulting in a memory leak. This behavior violates the expected resource management principles and can lead to gradual memory exhaustion over time. The problem becomes more severe when the rx-vlan-filter state is toggled multiple times during the device lifecycle, as the reference count tracking becomes inconsistent and can trigger kernel BUG_ON conditions.
The technical flaw manifests in the net/8021q/vlan.c file where the unregister_vlan_dev function performs a critical check that fails when the VLAN 0 information has been prematurely freed due to incorrect reference count management. This condition is classified under CWE-476 as a NULL pointer dereference and can be mapped to ATT&CK technique T1059.006 for kernel-level privilege escalation through kernel memory corruption. The vulnerability occurs because the system does not maintain a clear record of whether VLAN 0 was automatically added during device up events, leading to inconsistent cleanup routines during device down events.
The operational impact of this vulnerability is significant for systems relying on dynamic VLAN configuration and bond interfaces. An attacker could exploit this to cause denial of service through memory exhaustion or trigger kernel panics that result in system crashes. The vulnerability is particularly dangerous in environments where network interfaces are frequently brought up and down, such as in virtualized environments or systems using dynamic network configuration. The memory leak aspect makes this issue especially concerning for long-running systems, while the potential for kernel panics affects system stability and availability.
Mitigation strategies include updating to a patched kernel version that implements proper reference counting logic for VLAN 0 handling during runtime toggling of rx-vlan-filter. Administrators should monitor network interface lifecycle events and avoid frequent toggling of VLAN filtering features when possible. The fix ensures that VLAN 0 tracking maintains explicit state information about automatic additions during NETDEV_UP events, allowing consistent deletion during NETDEV_DOWN regardless of the current rx-vlan-filter state. System administrators should also implement monitoring for memory leaks in network subsystems and consider implementing automated restart procedures for affected services to maintain system availability.