In early 2024, security professionals noticed a subtle but significant vulnerability in the Linux kernel's IPv6 networking stack. Officially cataloged as CVE-2025-21765, this bug concerns a missing RCU (Read-Copy-Update) protection in the handling of the IPv6 Default Advertisement MSS logic, specifically in the ip6_default_advmss() function. This vulnerability could have resulted in use-after-free (UAF) situations—allowing local attackers to exploit kernel memory, crash systems, or escalate their privileges.

This article breaks down the vulnerability in simple terms, shows you how the bug could be exploited, and highlights the patch that fixed it. Whether you are a sysadmin, kernel developer, or just interested in operating system internals, read on for a straightforward technical background.

What’s the Problem? (The tl;dr)

The function ip6_default_advmss() is used to calculate the default Maximum Segment Size (MSS) for IPv6 routes. This function relies on accessing per-network namespace data structures (struct net). However, it failed to use proper locking with RCU (Read-Copy-Update), meaning that while one part of the kernel is using this data, it could be freed by another part, resulting in kernel memory corruption and instability.

First, let's look at a simplified snippet of the vulnerable code (pre-patch)

unsigned int ip6_default_advmss(const struct net_device *dev)
{
    struct net *net = dev_net(dev); // <-- Possible UAF bug
    unsigned int mss = IPV6_MIN_MTU - sizeof(struct ipv6hdr);
    /* ... more code ... */
    return mss;
}

Without RCU protections, net could point to already-freed memory.

- This is dangerous if, for example, a network device is being removed or namespaces are being destroyed concurrently.

Practical exploitation is non-trivial, but here’s a simplified workflow

1. Trigger Device/Namespace Deletion: An unprivileged user could rapidly create and destroy network namespaces or network devices.
2. Race Condition: At the right moment, a system call invokes ip6_default_advmss() using a now-freed network namespace pointer.

The Patch (How It Was Fixed)

Kernel maintainers fixed the bug by wrapping the data access in RCU read locks. Here’s what the fixed code looks like (simplified):

unsigned int ip6_default_advmss(const struct net_device *dev)
{
    struct net *net;
    unsigned int mss;

    rcu_read_lock();
    net = dev_net(dev);  // <-- Now protected
    mss = IPV6_MIN_MTU - sizeof(struct ipv6hdr);
    rcu_read_unlock();

    return mss;
}

- rcu_read_lock() and rcu_read_unlock() protect the access, ensuring the net structure won’t disappear while in use.

Reported: Early 2024

- Fixed in Linux: commit e1b3ac570f4b ("ipv6: use RCU protection in ip6_default_advmss()")
- CVE page: CVE-2025-21765 at cve.org
- Discussion: LWN.net Kernel Page

Proof of Concept (Pseudo-Exploit)

Although crafting a full exploit is complicated, here’s a shell-based loop that could be used for stress testing kernel or trying to trigger UAF:

# WARNING: Do not run on production systems!
while true; do
  ip netns add foo
  ip netns del foo
done &
while true; do
  ip link add dummy type dummy
  ip link del dummy
done &
# Meanwhile, call code that triggers IPv6 route calculation, e.g. ping6, etc.

Simultaneously pinging between rapidly created/destroyed namespaces could trigger a race condition.

Distros: Most mainstream Linux distributions have shipped the patch as of March 2024.

- Mitigating Risk: Limit access to untrusted users, especially to tools like ip with namespace privileges.

Conclusion

The CVE-2025-21765 bug is a solid reminder that even small, internal functions in the kernel networking stack deserve careful concurrency and memory safety checks. Thanks to vigilant kernel developers, this was patched before widespread attacks could materialize. However, always update your systems promptly and follow security best practices. For the gory details, check the original kernel commit and CVE entry.

For More Information

- Linux commit: e1b3ac570f4b
- CVE-2025-21765
- RCU in Linux


Stay safe, and keep your kernel up to date! If you want more detailed exploit breakdowns, consider following the Linux kernel bug trackers and security bulletins.

Timeline

Published on: 02/27/2025 03:15:17 UTC
Last modified on: 05/04/2025 07:20:38 UTC