In early 2024, security researchers and developers identified a race condition in the Linux kernel's handling of network interrupts for the HNS3 driver, affecting certain Huawei network cards. This vulnerability, now tracked as CVE-2025-21651, could cause system instability or kernel warnings by allowing interrupts to fire before the service tasks are ready to process them.

This exclusive post explains how the bug arises, its potential impact, the technical details including code snippets, and provides practical information on how it can be triggered and mitigated.

What Is CVE-2025-21651?

CVE-2025-21651 refers to an issue in the Linux kernel's *hns3* Ethernet driver (used for networks on some Huawei cards), where the "misc" interrupt vector (for miscellaneous interrupts) could be enabled at the wrong time. The bug occurs between enabling the interrupt and initializing the associated service task. If an interrupt is received during this window, it may trigger kernel warnings (Call Trace) or potentially confuse the workqueue mechanism.

Why Is This a Problem?

There is a race window—a brief period where interrupts are enabled, but the relevant tasks (that would process those interrupts) haven’t started yet. If the "misc" vector fires at that moment:

The kernel may try to schedule work on a non-initialized queue.

- This can cause warnings, errors, and possibly lead to more serious issues, like crashes, depending on system state and kernel config.

When this occurs, you might see a trace like the following in your kernel logs (dmesg)

[   16.324639] Call trace:
[   16.324641]  __queue_delayed_work+xb8/xe
[   16.324643]  mod_delayed_work_on+x78/xd
[   16.324655]  hclge_errhand_task_schedule+x58/x90 [hclge]
[   16.324662]  hclge_misc_irq_handle+x168/x240 [hclge]
...

This shows the kernel trying to schedule work before the underlying structure (workqueue) is initialized, resulting in a warning or trace.

In the kernel code (hns3 driver), something like this previously happened

err = request_irq(misc_irq, hclge_misc_irq_handle, ...);
if (err) {
    // Handle error
}
enable_irq(misc_irq);  // <-- Interrupt is enabled right after being requested

// Now initialize service task
init_delayed_work(&hw->service_task, service_task_fn);

If an interrupt is raised after enable_irq() but before init_delayed_work(), the handler might run in an uninitialized context.

The Fix: Don't Auto-Enable the Misc Interrupt

The patch (see upstream commit) changes the logic so the interrupt isn't auto-enabled when it's requested. Instead, it’s only enabled after the service task is set up and ready.

The Fixed Kernel Code (Pseudo)

// Request IRQ without auto-enable
err = request_irq(misc_irq, hclge_misc_irq_handle, IRQF_NO_AUTOEN, ...);
// Initialize the service task
init_delayed_work(&hw->service_task, service_task_fn);
// Now it's safe to enable the interrupt
enable_irq(misc_irq);

With this order, even if the interrupt fires immediately after being enabled, the service task will be ready to process it.

How would an attacker exploit this?

- Local, privileged access required: An attacker could stress or time network interface resets and carefully flood or trigger events that fire the misc IRQ right as the driver is initializing.
- Result: Kernel warnings, possible DoS (Denial of Service) condition if the kernel panics or disables network functionality as a safety measure.

Can this be exploited remotely?

- In most cases, remote exploitation is very unlikely, since it would require careful interaction as the network card is being initialized/reset.
- However, an attacker with the ability to repeatedly force network resets and conditions on a vulnerable machine (such as via local scripts or hijacked administrative access) could trigger instability.

References

- LKML Discussion on hns3 irq bug
- Official Patch Commit
- NVD Entry for CVE-2025-21651
- Huawei hns3 driver documentation

Practical Guidance

- Who is affected? Linux users running kernels using the *hns3* network driver—mainly certain Huawei network cards.

What should you do?

- Update your kernel to a version containing the fix (usually kernel 6.7+; see your distribution's advisory for a backport).
- Monitor kernel logs for suspicious hclge_misc_irq_handle traces if you cannot upgrade immediately.

Final Thoughts

CVE-2025-21651 is a classic, subtle kernel race condition: easy to overlook in code review, disruptive when triggered at the wrong time. While it isn’t directly a remote code execution or privilege escalation flaw, its ability to destabilize servers underscores the careful sequence required in low-level kernel code.

For system administrators, a simple kernel update is the fix. For kernel developers, it’s a lesson that, in interrupt-driven code, order matters.


*For live updates on this and similar vulnerabilities, track LKML security patches, NVD, and your distro’s security team.*


If you have questions about whether your system is affected, or how to prioritize this fix, drop a comment below!

Timeline

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