A new Linux kernel vulnerability, CVE-2024-42269, was recently addressed in the kernel’s netfilter subsystem. This issue centers on a possible null pointer dereference in the ip6table_nat_table_init() function, specifically when working with IPv6 NAT tables through iptables. If exploited, it could lead to a system crash or, under rare circumstances, other unpredictable behaviors.

In this exclusive deep dive, we break down the root cause, provide simplified code snippets, and review the patch that fixes the problem. If you're a system administrator or kernel developer, you’ll want to understand this issue — particularly if you rely on iptables for IPv6 NAT operations.

The Vulnerability in Simple Terms

The Linux kernel’s netfilter framework is the core engine behind iptables. It provides packet filtering, network address translation, and more. The problem described by CVE-2024-42269 happens inside ip6table_nat_table_init(), which initializes NAT tables for IPv6.

Accessing or dereferencing a NULL pointer leads to a kernel panic (crash).

The init function is made *available to userspace* (e.g., via tools like iptables when the module loads) before the kernel sets up all needed structures.

The registration order is wrong

xt_register_template(&ip6table_nat_table, &ip6table_nat_net_ops);
register_pernet_subsys(&ip6table_nat_net_ops);

The template is registered before the per-network subsystem is set up. That makes things accessible, but the backend isn’t ready yet.

If user code calls into this too early, you get a null pointer dereference.

The patch simply reverses the order, ensuring the subsystem is ready *before* userspace can reach it

- xt_register_template(&ip6table_nat_table, &ip6table_nat_net_ops);
- register_pernet_subsys(&ip6table_nat_net_ops);
+ register_pernet_subsys(&ip6table_nat_net_ops);
+ xt_register_template(&ip6table_nat_table, &ip6table_nat_net_ops);

Here's a simplified version showing the correct sequence

int __init ip6table_nat_init(void)
{
    int ret;

    // Initialize the per-network subsystem first!
    ret = register_pernet_subsys(&ip6table_nat_net_ops);
    if (ret)
        return ret;

    // Only then, register the xt template
    ret = xt_register_template(&ip6table_nat_table, &ip6table_nat_net_ops);
    if (ret)
        unregister_pernet_subsys(&ip6table_nat_net_ops);

    return ret;
}

Key principle: Make sure back-end kernel infrastructure is ready *before* exposing operability to userspace.

Any Linux system using the vulnerable version of netfilter and IPv6 NAT (ip6table_nat).

- Unprivileged users could potentially trigger the bug if they find a way to interact with iptables before init completes convincingly, though usually triggering would require root.

What Could an Attacker Do?

- Trigger a kernel panic: By sending commands to iptables for IPv6 NAT immediately after module load, but before initialization is complete, a race condition could cause the kernel to dereference a null pointer.
- Denial of Service: Most likely result is a full system crash and reboot — potentially a local DoS if the setup is automated, such as in containerized environments where ip6tables rules are scripted.

Example Exploit Scenario (Illustrative)

Suppose an attacker has rapid access to run ip6tables commands right after the module is loaded, possibly in a misconfigured container:

# Theoretical race trigger - in practice, this is very timing dependent
modprobe ip6table_nat &
ip6tables -t nat -A POSTROUTING -j MASQUERADE

If ip6tables invoked above is processed before the subsystem is properly registered, it could hit the bug. The attacker would see a system crash, indicating potential null-ptr dereference.

Note: This is not a remote code execution vulnerability or a universal local privilege escalation on its own.

- Official Patch Commit
- CVE-2024-42269 at NIST NVD *(check for updated details)*
- Linux kernel netfilter Documentation
- Netfilter iptables project

Bottom Line and Recommendations

- Patch Up: This fix has been merged in mainline and stable trees. Update your kernel if you’re using ip6table_nat or IPv6 NAT with iptables, especially in automated or containerized environments.

Minimize Attack Surface: Don’t let untrusted users run iptables commands or load modules.

- Container Security: Container platforms that fire up NAT rules on boot may be at risk, especially if they reload modules or underlying systems rapidly.

This is a textbook lesson in secure kernel programming: only open interfaces when all supporting infrastructure is ready.


Stay safe, keep your kernels current, and always review security advisories for even rare bugs like null pointer dereferences!

Timeline

Published on: 08/17/2024 09:15:08 UTC
Last modified on: 08/19/2024 20:53:51 UTC