# CVE-2024-42239: bpf: Fail bpf_timer_cancel when callback is being cancelled

> A race condition in the Linux kernel BPF timer cancellation logic can cause a deadlock and system lockup when multiple timers cancel each other.

- **Severity:** Medium 5.5 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/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:** 7 Aug 2024
- **Updated:** 11 May 2026
- **Also known as:** AZL-47453, AZL-47604, BELL-CVE-2024-42239, DEBIAN-CVE-2024-42239, GHSA-ggh4-5hvc-554r, ROOT-OS-DEBIAN-12-CVE-2024-42239, ROOT-OS-UBUNTU-2204-CVE-2024-42239, UBUNTU-CVE-2024-42239

## What is CVE-2024-42239?

CVE-2024-42239 is a medium-severity vulnerability in Linux kernel, published on 7 Aug 2024. 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:

bpf: Fail bpf_timer_cancel when callback is being cancelled

Given a schedule:

timer1 cb			timer2 cb

bpf_timer_cancel(timer2);	bpf_timer_cancel(timer1);

Both bpf_timer_cancel calls would wait for the other callback to finish
executing, introducing a lockup.

Add an atomic_t count named 'cancelling' in bpf_hrtimer. This keeps
track of all in-flight cancellation requests for a given BPF timer.
Whenever cancelling a BPF timer, we must check if we have outstanding
cancellation requests, and if so, we must fail the operation with an
error (-EDEADLK) since cancellation is synchronous and waits for the
callback to finish executing. This implies that we can enter a deadlock
situation involving two or more timer callbacks executing in parallel
and attempting to cancel one another.

Note that we avoid incrementing the cancelling counter for the target
timer (the one being cancelled) if bpf_timer_cancel is not invoked from
a callback, to avoid spurious errors. The whole point of detecting
cur->cancelling and returning -EDEADLK is to not enter a busy wait loop
(which may or may not lead to a lockup). This does not apply in case the
caller is in a non-callback context, the other side can continue to
cancel as it sees fit without running into errors.

Background on prior attempts:

Earlier versions of this patch used a bool 'cancelling' bit and used the
following pattern under timer->lock to publish cancellation status.

lock(t->lock);
t->cancelling = true;
mb();
if (cur->cancelling)
	return -EDEADLK;
unlock(t->lock);
hrtimer_cancel(t->timer);
t->cancelling = false;

The store outside the critical section could overwrite a parallel
requests t->cancelling assignment to true, to ensure the parallely
executing callback observes its cancellation status.

It would be necessary to clear this cancelling bit once hrtimer_cancel
is done, but lack of serialization introduced races. Another option was
explored where bpf_timer_start would clear the bit when (re)starting the
timer under timer->lock. This would ensure serialized access to the
cancelling bit, but may allow it to be cleared before in-flight
hrtimer_cancel has finished executing, such that lockups can occur
again.

Thus, we choose an atomic counter to keep track of all outstanding
cancellation requests and use it to prevent lockups in case callbacks
attempt to cancel each other while executing in parallel.

Weakness: CWE-667 (Improper Locking)

## Which versions are affected by CVE-2024-42239?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2024-42239?

Apply the vendor's patch.

## Is CVE-2024-42239 being exploited?

There is no known exploitation of CVE-2024-42239 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-2024-42239 and fix it if it is affected.

What it is: CVE-2024-42239 is a medium-severity vulnerability in Linux kernel, published on 7 Aug 2024. 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-2024-42239 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-2024-42239/ (Markdown: https://insights.sbomify.com/CVE-2024-42239/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2024-42239
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2024-42239
- OSV: https://osv.dev/vulnerability/CVE-2024-42239

---
From sbomify Insights: https://insights.sbomify.com/CVE-2024-42239/
Summaries are generated and may contain mistakes; the official description above is the record.