# CVE-2025-39744: rcu: Fix rcu_read_unlock() deadloop due to IRQ work

> A flaw in the Linux kernel's RCU subsystem can cause a system hang or deadlock during IRQ exit, leading to denial of service.

- **Severity:** High 7.1 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 11 Sep 2025
- **Updated:** 11 May 2026
- **Also known as:** AZL-67253, AZL-70807, BELL-CVE-2025-39744, DEBIAN-CVE-2025-39744, GHSA-vqg7-j8rg-9mw8, ROOT-OS-DEBIAN-11-CVE-2025-39744, ROOT-OS-DEBIAN-12-CVE-2025-39744, ROOT-OS-UBUNTU-2204-CVE-2025-39744, ROOT-OS-UBUNTU-2404-CVE-2025-39744, UBUNTU-CVE-2025-39744

## What is CVE-2025-39744?

CVE-2025-39744 is a high-severity denial of service vulnerability in Linux kernel, published on 11 Sep 2025. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

In the Linux kernel, the following vulnerability has been resolved:

rcu: Fix rcu_read_unlock() deadloop due to IRQ work

During rcu_read_unlock_special(), if this happens during irq_exit(), we
can lockup if an IPI is issued. This is because the IPI itself triggers
the irq_exit() path causing a recursive lock up.

This is precisely what Xiongfeng found when invoking a BPF program on
the trace_tick_stop() tracepoint As shown in the trace below. Fix by
managing the irq_work state correctly.

irq_exit()
  __irq_exit_rcu()
    /* in_hardirq() returns false after this */
    preempt_count_sub(HARDIRQ_OFFSET)
    tick_irq_exit()
      tick_nohz_irq_exit()
	    tick_nohz_stop_sched_tick()
	      trace_tick_stop()  /* a bpf prog is hooked on this trace point */
		   __bpf_trace_tick_stop()
		      bpf_trace_run2()
			    rcu_read_unlock_special()
                              /* will send a IPI to itself */
			      irq_work_queue_on(&rdp->defer_qs_iw, rdp->cpu);

A simple reproducer can also be obtained by doing the following in
tick_irq_exit(). It will hang on boot without the patch:

  static inline void tick_irq_exit(void)
  {
 +	rcu_read_lock();
 +	WRITE_ONCE(current->rcu_read_unlock_special.b.need_qs, true);
 +	rcu_read_unlock();
 +

[neeraj: Apply Frederic's suggested fix for PREEMPT_RT]

Weakness: CWE-125 (Out-of-bounds Read)

## Which versions are affected by CVE-2025-39744?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2025-39744?

Apply the vendor's patch.

## Is CVE-2025-39744 being exploited?

There is no known exploitation of CVE-2025-39744 in the wild and no public exploit code.

## Other vulnerabilities in Linux kernel

- [CVE-2026-53266](https://insights.sbomify.com/CVE-2026-53266/): Linux Kernel Out-of-Bounds Write (high 8.8, exploited in the wild)
- [CVE-2013-6282](https://insights.sbomify.com/CVE-2013-6282/): Linux Kernel Improper Input Validation (high 8.8, exploited in the wild)
- [CVE-2022-0185](https://insights.sbomify.com/CVE-2022-0185/): Linux Kernel Heap-Based Buffer Overflow (high 8.4, exploited in the wild)
- [CVE-2013-2094](https://insights.sbomify.com/CVE-2013-2094/): Linux Kernel Privilege Escalation (high 8.4, exploited in the wild)
- [CVE-2026-53362](https://insights.sbomify.com/CVE-2026-53362/): Linux Kernel Unspecified (high 7.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2025-39744 and fix it if it is affected.

What it is: CVE-2025-39744 is a high-severity denial of service vulnerability in Linux kernel, published on 11 Sep 2025. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2025-39744 in the wild and no public exploit code.

Affected product: Linux kernel. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2025-39744/ (Markdown: https://insights.sbomify.com/CVE-2025-39744/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-39744
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-39744
- OSV: https://osv.dev/vulnerability/CVE-2025-39744

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-39744/
Summaries are generated and may contain mistakes; the official description above is the record.