CVE-2025-21681 openvswitch: fix lockup on tx to unregistering netdev with carrier
An attacker can cause a system lockup and require a reboot by sending traffic to an unregistering network device in the Linux kernel's Open vSwitch module.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2025-21681?
CVE-2025-21681 is a medium-severity denial of service vulnerability in Linux kernel, published on 31 Jan 2025. It can only be exploited from the same machine or local network, with no action from the victim.
Official description
In the Linux kernel, the following vulnerability has been resolved:
openvswitch: fix lockup on tx to unregistering netdev with carrier
Commit in a fixes tag attempted to fix the issue in the following sequence of calls:
do_output -> ovs_vport_send -> dev_queue_xmit -> __dev_queue_xmit -> netdev_core_pick_tx -> skb_tx_hash
When device is unregistering, the 'dev->real_num_tx_queues' goes to zero and the 'while (unlikely(hash >= qcount))' loop inside the 'skb_tx_hash' becomes infinite, locking up the core forever.
But unfortunately, checking just the carrier status is not enough to fix the issue, because some devices may still be in unregistering state while reporting carrier status OK.
One example of such device is a net/dummy. It sets carrier ON on start, but it doesn't implement .ndo_stop to set the carrier off. And it makes sense, because dummy doesn't really have a carrier. Therefore, while this device is unregistering, it's still easy to hit the infinite loop in the skb_tx_hash() from the OVS datapath. There might be other drivers that do the same, but dummy by itself is important for the OVS ecosystem, because it is frequently used as a packet sink for tcpdump while debugging OVS deployments. And when the issue is hit, the only way to recover is to reboot.
Fix that by also checking if the device is running. The running state is handled by the net core during unregistering, so it covers unregistering case better, and we don't really need to send packets to devices that are not running anyway.
While only checking the running state might be enough, the carrier check is preserved. The running and the carrier states seem disjoined throughout the code and different drivers. And other core functions like __dev_direct_xmit() check both before attempting to transmit a packet. So, it seems safer to check both flags in OVS as well.
Weakness: CWE-835
Which versions are affected by CVE-2025-21681?
It affects 1 product.
Products include Linux kernel.
How do I fix CVE-2025-21681?
Apply the vendor's patch.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2025-21681 being exploited?
There is no known exploitation of CVE-2025-21681 in the wild and no public exploit code.
Other vulnerabilities in Linux kernel
Sources
Also known as AZL-57055, BELL-CVE-2025-21681, DEBIAN-CVE-2025-21681, GHSA-2x4f-r6q9-v77h, UBUNTU-CVE-2025-21681.