Summary:
On recent Linux kernel versions, a WiFi configuration bug existed (CVE-2024-57898) — leading to potential system issues and stability risks in modern wireless networking. This post explains what happened, why it happened, how it got fixed, and whether it was exploitable. If you maintain Linux systems or build drivers, you’ll want to know about this.

[Potential Impact](#potential-impact)

7. [Proof-of-Concept/Exploit Details](#proof-of-conceptexploit-details)

What is CVE-2024-57898?

CVE-2024-57898 is a vulnerability in the Linux kernel’s wifi subsystem (cfg80211). During the WiFi link deletion process, code would clear a link ID from an internal bitmap before all the clean-up functions were done.

Some clean-up functions (like cfg80211_cac_event()) *expected* that link ID to still exist in the bitmap! This caused warnings, could break user networks, and (in theory) could allow a malicious user or buggy software to trigger kernel instability.

When you connect to a WiFi network in Linux, a complex stack comes into play. Functions track “links” between your device and wireless networks. These links are managed using structures and bitmaps, like so:

But what if you remove a link ID from the bitmap too soon? That’s what this bug is all about.

The Bug: Order of Operations Mistake

When deleting a link, the kernel *first* removed the link ID from the bitmap (valid_links), and *then* tried to clean up everything related to that link.

But:
- Some clean-up code (specifically, cfg80211_cac_event()) still needed to know the link ID was valid (i.e., present in the bitmap).
- Removing the link ID too soon tripped sanity checks (see WARN_ON()), and could, in some situations, cause instability.

The call chain looks like this, with the problem step highlighted

nl80211_remove_link()
    cfg80211_remove_link()
        ieee80211_del_intf_link()
            ieee80211_vif_set_links()
                ieee80211_vif_update_links()
                    ieee80211_link_stop()
                        cfg80211_cac_event()   <-- needs link ID to be present

But the bitmap is cleared before cfg80211_cac_event() is called!

Code Snippet: The Vulnerable Function

Here’s a simplified and annotated pseudocode representation (the real code is in net/wifi/cfg80211/link.c):

// Old (vulnerable) order:
void cfg80211_remove_link(struct wiphy *wiphy, struct wireless_dev *wdev, int link_id) {
    clear_bit(link_id, &wdev->valid_links);      // Step 1: Remove from bitmap
    perform_cleanup(link_id, wdev);              // Step 2: Clean up
    // ... rest of code
}

The Correct, Fixed Order

// New (secure) order:
void cfg80211_remove_link(struct wiphy *wiphy, struct wireless_dev *wdev, int link_id) {
    perform_cleanup(link_id, wdev);              // Step 1: Clean up
    clear_bit(link_id, &wdev->valid_links);      // Step 2: Now remove from bitmap
    // ... rest of code
}

How the Fix Works

The fix:
*Only clear the link ID from the bitmap after all clean-up functions have successfully run.*

The kernel no longer triggers sanity-check warnings or corrupts state during deletion.

Commit message:
> "Clear the link ID from the bitmap only after we’ve completed the link clean-up."

(See original patch commit for full details.)

Unexpected crashes in rare conditions.

- Possibly opening the door to DoS (denial of service) exploits if an attacker could force rapid/additional link deletions.


## Proof-of-Concept / Exploit Details

Exploitation risk:
While this bug *could* be triggered by local code (or a privileged user) exploiting rapid or malformed WiFi interface changes, no known remote exploits exist.

You might reproduce a kernel warning with a script like the following (you need admin)

# WARNING: This may crash, hang, or cause kernel errors in vulnerable kernels!
iw dev wlan interface add test type managed
iw dev test link add 1
iw dev test link del 1    # During this, system may throw WARN_ON or log errors

You’d want to monitor dmesg or the kernel log for warnings that mention cfg80211_cac_event or WARN_ON.

Original patch and discussion:

Kernel.org Commit: wifi: cfg80211: clear link ID from bitmap during link delete after clean up

CVE Record:

NVD listing (link may not be live yet)

Linux cfg80211 Documentation:

Linux Wireless Wiki
Linux Wireless Source Tree

Conclusion

CVE-2024-57898 is an example of how even small kernel programming mistakes — like the *order* in which you remove flags — can cause instability, warnings, and possible security risks. The Linux development team responded quickly and fixed the bug, so make sure to upgrade to a kernel release with the patch, especially if you run new WiFi hardware or experimental multi-link setups.

Timeline

Published on: 01/15/2025 13:15:14 UTC
Last modified on: 05/04/2025 10:06:09 UTC