Summary:
A recent Linux Kernel vulnerability identified as CVE-2025-21648 exposed a risk in the netfilter conntrack hashtable resizing logic. This post breaks down what happened, the fix, and how it could have been abused—explained in simple, clear language.

What Is CVE-2025-21648 All About?

The Linux Kernel’s netfilter conntrack is a module that keeps track of network connections for firewall and NAT. To manage these entries, it uses an in-memory hashtable. Properly sizing this hashtable is critical—if it grows too large, the kernel might try to allocate more memory than is safe.

With CVE-2025-21648, the problem was that the maximum allowed hashtable size wasn't properly capped. That meant, under some edge cases, the kernel could attempt to allocate more memory than INT_MAX (the largest positive integer for the system)—which isn’t safe or intended.

The vulnerability is linked to this kernel warning

> "mm: Consider __GFP_NOWARN flag for oversized kvmalloc() calls"

When resizing the conntrack hashtable (basically, making space for more connections), the kernel could hit this warning if the allocation was huge.

Technical Details and Original Fix

The offending logic was fixed by capping the hashtable size to INT_MAX, which is the maximum value an integer can hold on the platform. Anything bigger is simply refused.

From the vulnerability fix

#include <linux/limits.h> // For INT_MAX

unsigned long max = totalram_pages / divisor;
if (max > INT_MAX)
    max = INT_MAX;

This simple change means the kernel will now never try to allocate a conntrack hashtable larger than it can safely handle.

Important note:
Hashtable resize is only possible from the initial network namespace (init_netns). This means regular, unprivileged users or containers running in their own namespaces are not affected—the attack surface is somewhat limited.

IF the hashtable maximum size isn’t capped

- AND a privileged user triggers conntrack resize (e.g., via /proc/sys/net/netfilter/nf_conntrack_buckets)

A minimal pseudo-exploit (with root privileges) might look like

# Dangerous! DO NOT RUN ON PRODUCTION SYSTEMS
echo 2147483648 > /proc/sys/net/netfilter/nf_conntrack_buckets

Stability Problems: The system could become unstable if kernel warnings pile up

However, because the resize is only accessible in the init_netns (the initial, privileged network namespace), normal users or attackers without _root_ cannot trigger this scenario.

Kernel Patch & Commit:

netfilter: conntrack: clamp maximum hashtable size to INT_MAX

"mm: Consider __GFP_NOWARN flag for oversized kvmalloc() calls"

Official CVE entry:

CVE-2025-21648 at cve.org (check for publication)


## What Should Linux Sysadmins/Developers Do?

Don’t mess with conntrack settings unless necessary

- For production firewalls using conntrack: check the value of /proc/sys/net/netfilter/nf_conntrack_buckets and keep it reasonable

Final Words

CVE-2025-21648 was a _potentially dangerous but easily patched_ oversight in Linux’s netfilter module. It offered a new way for root or privileged attackers to squirm into trouble by exhausting kernel memory. Thanks to quick patching, the hashtable size is now safely clamped, and your Linux firewalls are better protected.


*You’ve read this explanation only here. For more security breakdowns, stay tuned!*

Timeline

Published on: 01/19/2025 11:15:10 UTC
Last modified on: 05/04/2025 07:18:12 UTC