CVE-2021-47555: net: vlan: fix underflow for the real_dev refcnt

Severity: medium (CVSS 5.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.2% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2024-05-24 · updated 2026-08-04
Weakness: CWE-191

Description

In the Linux kernel, the following vulnerability has been resolved: net: vlan: fix underflow for the real_dev refcnt Inject error before dev_hold(real_dev) in register_vlan_dev(), and execute the following testcase: ip link add dev dummy1 type dummy ip link add name dummy1.100 link dummy1 type vlan id 100 ip link del dev dummy1 When the dummy netdevice is removed, we will get a WARNING as following: ======================================================================= refcount_t: decrement hit 0; leaking memory. WARNING: CPU: 2 PID: 0 at lib/refcount.c:31 refcount_warn_saturate+0xbf/0x1e0 and an endless loop of: ======================================================================= unregister_netdevice: waiting for dummy1 to become free. Usage count = -1073741824 That is because dev_put(real_dev) in vlan_dev_free() be called without dev_hold(real_dev) in register_vlan_dev(). It makes the refcnt of real_dev underflow. Move the dev_hold(real_dev) to vlan_dev_init() which is the call-back of ndo_init(). That makes dev_hold() and dev_put() for vlan's real_dev symmetrical.

Affected products

Linux kernel: from 5.4.160 before 5.4.163; from 5.10.80 before 5.10.83; from 5.14.19 before 5.15; from 5.15.3 before 5.15.6; 5.16-rc1, 5.16-rc2 — fixed in 5.10.83, 5.15, 5.15.6, 5.4.163

How to fix

Upgrade to a fixed version: Linux kernel 5.10.83, 5.15, 5.15.6, 5.4.163. Check the vendor advisory for mitigations.

References

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