CVE-2025-21639 - Fixing a Linux Kernel SCTP Sysctl Vulnerability (Explained Simply)

A security vulnerability labeled CVE-2025-21639 was recently *resolved* in the Linux kernel. This flaw affected the way the kernel handled SCTP (Stream Control Transmission Protocol) sysctl parameters, specifically the rto_min and rto_max values. The issue revolved around bad usage of the current->nsproxy pointer, which could result in system crashes under certain conditions.

This post breaks down what happened, why it mattered, and how developers fixed it — using simple language and exclusive insights.

What Is SCTP and Sysctl?

SCTP is a transport protocol like TCP or UDP, used in Linux for certain communications. sysctl is a Linux mechanism that lets you tweak kernel settings at runtime — this includes SCTP timeouts like rto_min (minimum retransmission timeout) and rto_max.

The Old Problematic Approach

Previously, when code needed to access SCTP sysctl parameters, it referred to the network namespace via the current->nsproxy pointer.

struct net *net = current->nsproxy->net_ns;

This net structure contains various subsystem settings, including SCTP parameters

value = net->sctp.rto_min;

Inconsistencies could arise about which network namespace you were working in.

If current->nsproxy was NULL, accessing its members caused a crash (kernel Oops). A tool called syzbot found this bug by simulating weird exit conditions with acct(2).

The Security Impact

- A malicious (or even regular) user could trigger a kernel crash simply by manipulating processes and network sysctl settings in the right way.

The core of the fix is this

- Stop using current->nsproxy, and instead get the relevant net structure directly from the sysctl table’s data field.

Before (Dangerous)

int get_rto_min(struct ctl_table *table, int *val) {
    struct net *net = current->nsproxy->net_ns;
    *val = net->sctp.rto_min;
    return ;
}

After (Safe)

int get_rto_min(struct ctl_table *table, int *val) {
    struct net *net = container_of(table->data, struct net, sctp.rto_min);
    *val = net->sctp.rto_min;
    return ;
}

Or, taking only what's necessary, just

*val = *(int *)(table->data);

This makes sure the net structure used always matches the one associated with the sysctl call, not the caller’s process.

Exploiting the Vulnerability (for Understanding)

While not providing a ready-to-use exploit (which would be dangerous here), here's a description of how one could have triggered the bug:

Make the process exit quickly, or use acct(2) to force exit in a weird state.

- If the kernel tried to access SCTP sysctl values during process teardown, current->nsproxy could be NULL, resulting in a kernel crash.

Example Simulated Attack Code

#include <sys/acct.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/syscall.h>

int main() {
    syscall(SYS_acct, "/dev/null"); // Triggers syzbot's scenario
    // Now, try to access SCTP sysctls here ...
    system("sysctl -w net.sctp.rto_min=100");
    return ;
}

*(This is illustrative; actual exploitation would be race-condition dependent and likely require heavy fuzzing tools.)*

References & Further Reading

- Linux kernel patch commit (fixes CVE-2025-21639)
- Syzbot report (search for SCTP sysctl bugs)
- Kernel sysctl documentation
- Intro to SCTP on Linux
- Container_of macro explained

Summary

CVE-2025-21639 highlights how even seemingly minor kernel pointer mistakes can cause critical security issues, including DoS vulnerabilities. The Linux team quickly fixed the bug by moving from a user-context-dependent pointer access to a safe, context-independent method, making Linux SCTP sysctl handling robust and consistent.

Keep your kernels updated — and always be wary of context-sensitive pointers!


*Have questions or want more details? Let us know below!*

Timeline

Published on: 01/19/2025 11:15:09 UTC
Last modified on: 02/27/2025 22:01:10 UTC