This article will walk you through CVE-2025-21646—a vulnerability in the Linux kernel's kafs filesystem relating to cell name length overruns. We’ll break down the bug, how it could be triggered, show you code snippets, and reference upstream sources. All in straightforward, American English.

What Is AFS and Why the Cell Name Length Matters

AFS (Andrew File System) is a distributed file system mostly used in academic and enterprise environments. The kafs module in Linux handles the client side for AFS. One of its tasks is to manage "cells" (which are essentially AFS domains)—each with a unique name.

The buggy versions of kafs let you create a cell name up to 256 bytes, but that runs into a Linux procfs constraint, which limits file or directory names to 255 bytes. The procfs is where proc-related details are exposed, often under /proc/net/afs/.

What Went Wrong?

kafs let you register a cell name up to its internal limit (256 bytes). When it tries to create a directory in /proc/net/afs/[cell name], the kernel panics because procfs filenames max out at 255 bytes. To make it worse, DNS only allows up to 253 bytes (excluding counts/null-terminators).

If you managed to register a 256-byte cell, creating the corresponding procfs directory fails with a CPU warning and stack trace, causing instability or potential denial of service.

Sample warning

WARNING: CPU:  PID: 9 at fs/proc/generic.c:405

Here’s how the old code was accepting 256 bytes

#define AFS_MAXCELLNAME   256
char cellname[AFS_MAXCELLNAME];
// ... reads cell name of up to 256 bytes

The improved code (commit)

#define AFS_MAXCELLNAME   253  // NEW: strictly limit to 253

The check for YFS (a related protocol) is also split, allowing up to the protocol’s 256 but ignoring those over 253 for mount points.

Exploit: How Could It Be Triggered?

1. Create a cell with a name 256 bytes long using kafs tools or direct interaction with kernel APIs.
2. The kernel will try to make /proc/net/afs/[cell name], but that fails—procfs blocks it at 255. This triggers a kernel warning and may cause other errors, instability, or DoS in some contexts (depends on configuration and how strictly errors are handled).

NOTE: This mostly affects systems that allow unprivileged users to register arbitrary AFS cells, which is rare, but misconfigured systems exist.

Let’s say you’re writing a test tool in C to register an overlong cell

#include <stdio.h>
#include <string.h>
#include <sys/ioctl.h>
#include <fcntl.h>

int main() {
    int fd = open("/proc/net/afs", O_RDWR);
    if (fd == -1) {
        perror("afs open");
        return 1;
    }
    char longcell[256 + 1];
    memset(longcell, 'A', 256);
    longcell[256] = ;

    // pretend ioctl to register cell, depending on kernel API
    // ioctl(fd, AFS_REGISTER_CELL, &longcell);

    printf("Registered cell name: %s\n", longcell);
    close(fd);
    return ;
}

This code won’t do anything unless you have the right permissions and kernel API, but shows the concept.

The maintainers changed AFS’s cell name maximum to 253 bytes. Now, it

- Never allows creation of cell names over 253 bytes, making sure all kernel structures, filesystems, and DNS handling stay in sync.
- Separately, for the YFS protocol, it still fetches up to 256 bytes, but ignores cells longer than 253 in context for mount and procfs.

Fix commit:
- afs: Fix the maximum cell name length

Kernel mailing list post:
- https://lore.kernel.org/lkml/20240604084853.2919147-1-dhowells@redhat.com/

What Should You Do?

- Update your Linux kernel to a version that includes this fix (mainline patch accepted June 2024, see above).
- Audit systems: If you use AFS and allow users to register their own cells, check for excessive cell name lengths.
- Review configs: Make sure unprivileged users can’t register arbitrary AFS cells, unless you know what you’re doing.

CVE-2025-21646 is a Linux kernel bug.

- It allowed overly long AFS cell names—but the underlying filesystems/discovery layers can’t handle them.
- Could trigger kernel warnings, potential stability/DoS risk.

References

- Official patch on GitHub
- Kernel mailing list discussion
- procfs documentation

*Always keep your kernel updated! For more in-depth kernel security news, follow your distro’s channels.*

Timeline

Published on: 01/19/2025 11:15:10 UTC
Last modified on: 11/03/2025 21:19:00 UTC