---

Linux kernel vulnerabilities are always critical because they can affect a wide array of systems — from personal laptops to massive servers. The recently patched CVE-2025-21764 brings attention to a subtle yet severe bug in the Neighbor Discovery Protocol implementation in the kernel’s networking stack. If you rely on Linux, especially in networks that use IPv6, keep reading for a non-technical breakdown, example code, and what steps you need to take.

What is CVE-2025-21764?

Reported and fixed in early 2024, CVE-2025-21764 is a use-after-free (UAF) vulnerability that affects the neighbor discovery cache used by IPv6 networking. The issue lives inside the function ndisc_alloc_skb(), which is responsible for allocating and preparing network packets (SKBs) for NDISC, part of the IPV6 stack.

The Specific Problem

Normally, concurrency safeguards like RCU (Read, Copy, Update) or RTNL (the network stack's locking mechanism) make sure that data is not freed while still in use, by any part of the kernel. But here, ndisc_alloc_skb() could be called *without* either of those safety nets, opening the door for a classic race condition.

This means: If something else frees memory while this function is working on it, the code could accidentally access memory that’s been reallocated for something else. This is the textbook definition of a "use-after-free" — and a critical target for attackers, who might exploit it to execute code or crash the system.

Let’s do a simplified version. Here’s what kind of logic was there before the patch

// Before patch — no RCU protection!
struct sk_buff *ndisc_alloc_skb(struct net_device *dev, ...) {
    struct neighbour *neigh = __neigh_lookup(...);
    if (!neigh) return NULL;

    // ... work with 'neigh' as if nothing can change it

    // Potentially, 'neigh' could be freed by someone else!
    // UAF Vulnerability.
    ...
}

Here’s a fixed version, which includes RCU locking to prevent simultaneous modification

// After patch — adding RCU protection
struct sk_buff *ndisc_alloc_skb(struct net_device *dev, ...) {
    struct neighbour *neigh;

    rcu_read_lock(); // <--- Lock here

    neigh = neigh_lookup(...);
    if (!neigh) {
        rcu_read_unlock();
        return NULL;
    }

    // Now, any access to 'neigh' is safe while RCU is held
    ...
    rcu_read_unlock(); // <--- Unlock at the end
}

RCU (Read, Copy, Update) allows safe, concurrent access to shared data structures. Now, even if the neighbor entry is being modified elsewhere, it won’t disappear out from under ndisc_alloc_skb(). This prevents the use-after-free exploit.

Theoretically, an attacker with local access (maybe running a malicious app or process) could

1. Trigger Neighbor Discovery by sending an IPv6 packet that requires a new neighbor entry to be processed.

Race the kernel by rapidly freeing or modifying neighboring data structures.

3. Cause the kernel to use memory that’s already been freed — potentially leading to a crash (Denial of Service), or with further manipulations, arbitrary code execution in kernel mode.

While races like this are tricky to win on demand, technologies like kernel fuzzers (such as syzkaller) have made finding and weaponizing these bugs much easier.

Proof-of-Concept (POC) Outline

While no public ready-to-run exploit has appeared as of writing, here’s how a POC might look in principle:

# Pseudo-code — not a working exploit
import socket, threading

def flood_neighbors(iface):
    # repeatedly send IPv6 packets from new source addresses to trigger neighbor cache creation/freeing
    ...

threading.Thread(target=flood_neighbors, args=('eth',)).start()
# Meanwhile, trigger the vulnerable function via normal IPv6 operations
...

Upstream patch:

linux.git - net: ndisc: use RCU protection in ndisc_alloc_skb()

CVE entry:

CVE-2025-21764 at NVD

Discussion:

LKML mailing list thread with patch discussion

How to Stay Safe

- Update your kernel. All major Linux distributions are backporting this fix rapidly. For upstream, the patch to look for is titled _“ndisc: use RCU protection in ndisc_alloc_skb()”_.
- Protect unprivileged users. Don’t allow untrusted code on critical systems, and use containerization or virtualization to limit kernel exposure.
- Reduce kernel attack surface. If you don’t use IPv6, consider disabling it or hardening your kernel configuration.

Conclusion

CVE-2025-21764 is another reminder that concurrency bugs are a perennial enemy in OS development, and that modern exploitation tech means even "minor" data races can have serious security consequences. The prompt response by Linux kernel maintainers once again kept users ahead of attackers, but keeping updated is key.

Stay patched, stay alert — and happy (secure) hacking!

*This write-up is exclusive and simplified for all Linux users and sysadmins. For more technical analysis, always check the original patch and mailing list references.*

Timeline

Published on: 02/27/2025 03:15:17 UTC
Last modified on: 05/04/2025 07:20:36 UTC