The Linux kernel is trusted by millions of users and organizations, but even mature drivers like the ath9k_htc WiFi module can have tricky vulnerabilities. In early 2024, researchers and fuzzers identified CVE-2024-26897, a race condition in the initialization sequence of the ath9k_htc subsystem, potentially leading to system crashes or privilege escalation. Here, we'll explain what went wrong, how it was exploited, and how it was fixed—with code snippets and links to the official resources.

The Bug in Simple Terms

The bug affects the Atheros 802.11n HTC based WiFi USB driver (ath9k_htc). The problem starts with how the WiFi device is initialized on connection:

The ath9k_htc_probe_device() function sets up hardware and data structures.

- The USB subsystem exposes the device to the kernel and triggers events even before the driver's init is fully done.
- The problematic part is the ath9k_wmi_event_tasklet() function, which handles events and assumes all parts of the device are ready.

*But sometimes, an event handler is called before everything is set up—leading to reads of uninitialized memory or dereferencing NULL pointers.*

Syzbot, a fuzzing robot, was able to hit this race, causing system instability.

Where's the Race Condition?

The problem lies in the timing: a tasklet (a part of the kernel handling deferred work) can run before initialization is complete. Take a look at this (simplified) flow:

void ath9k_htc_probe_device(struct usb_interface *interface) {
    // ... Setup data structures ...
    // <--- Race: events can arrive here!
    ath9k_tx_init(); // Old fix only covered TX events
    // ...
    device->initialization_complete = true; // (This was too late!)
}

Meanwhile, the event handler does something like

void ath9k_wmi_event_tasklet(unsigned long data) {
    struct ath9k_htc_device *dev = (ath9k_htc_device *)data;

    // Assumes dev and inner structures are fully initialized!
    if (!dev->tx_handle->wmi_txstatus_eventid) {
        // Oops: crash if tx_handle is NULL!
    }

    // process event...
}

If a WMI event fires before the probe finished, you risk referencing invalid or uninitialized pointers.

Previous (Partial) Fix

A prior patch commit 8b3046abc99e tried to solve this for just one specific event (TXSTATUS). It set a flag *after* initializing one structure, aborting the event early if not ready.

But syzbot showed this wasn't enough: other events could also trigger the race, leading to other types of crashes!

The Full Fix — Now Covering All Events

The real, comprehensive fix is to move the "initialization complete" check out of just the TX path, and place it over the entire tasklet. Now, no event whatsoever is processed until the driver and all its data structures are set up.

Key Patch Snippet

// Inside ath9k_wmi_event_tasklet() - new code
void ath9k_wmi_event_tasklet(unsigned long data)
{
    struct ath9k_htc_priv *priv = (struct ath9k_htc_priv *)data;

    // Prevent tasklet execution until the device is fully initialized
    if (!READ_ONCE(priv->is_initialized)) {
        return;
    }

    // ... proceed to process events safely ...
}

And in probe (after *all* init is complete)

int ath9k_htc_probe_device(...){
    ...
    // All structures are now ready
    WRITE_ONCE(priv->is_initialized, true);
    ...
}

No events will be processed until is_initialized is set!

Reference (fix commit):
- Upstream Kernel Patch Discussion
- Commit in Linus' tree

How could an attacker exploit this bug?

- Attackers with physical/local access could insert or reset a USB WiFi card at critical times, racing the kernel's initialization and the USB event pipeline.
- Malicious or buggy devices/emulators could trigger unexpected event payloads during initialization.
- Result: Kernel Oops (panic), potential NULL dereference, and—on rare occasions—privilege escalation if the attack is combined with heap spraying or other techniques to place attacker-controlled data at predictable locations.

A simplified reproducer (not a full exploit)

# Use USB emulation or fuzzing to attach and detach ath9k_htc device rapidly,
# injecting crafted WMI events milliseconds after the device appears, but before
# kernel initialization is complete.

Syzbot's report:
- Example syzbot crash report

Update your kernel ASAP if you use USB Atheros WiFi dongles (ath9k_htc).

- Check your distro's advisories: Most mainstream distributions will have detailed security advisories.
- Do not use untrusted USB devices — as with all hardware, bad devices can get creative with driver bugs.

Extra References

- CVE-2024-26897 at Mitre
- Relevant kernel source code
- ath9k_htc driver documentation

In Conclusion

CVE-2024-26897 is an example of why kernel initialization order and race conditions matter—especially with hotplug hardware like WiFi dongles. Thanks to fuzzers like syzbot and quick kernel developer response, a subtle but dangerous race has been fixed for good. Always update, don't trust suspicious USB devices, and keep an eye out for similar initialization bugs in other drivers!

Timeline

Published on: 04/17/2024 11:15:10 UTC
Last modified on: 05/04/2025 12:55:07 UTC