# CVE-2025-68209: mlx5: Fix default values in create CQ

> A null pointer dereference in the mlx5 kernel driver allows a local attacker to cause a system crash via specific completion queue interactions.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 16 Dec 2025
- **Updated:** 21 Oct 2026
- **Also known as:** AZL-72517, BELL-CVE-2025-68209, DEBIAN-CVE-2025-68209, GHSA-rcqh-q2vf-rp2f, ROOT-OS-DEBIAN-12-CVE-2025-68209, ROOT-OS-DEBIAN-13-CVE-2025-68209, ROOT-OS-UBUNTU-2404-CVE-2025-68209, UBUNTU-CVE-2025-68209

## What is CVE-2025-68209?

CVE-2025-68209 is a vulnerability in Linux, published on 16 Dec 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:

mlx5: Fix default values in create CQ

Currently, CQs without a completion function are assigned the
mlx5_add_cq_to_tasklet function by default. This is problematic since
only user CQs created through the mlx5_ib driver are intended to use
this function.

Additionally, all CQs that will use doorbells instead of polling for
completions must call mlx5_cq_arm. However, the default CQ creation flow
leaves a valid value in the CQ's arm_db field, allowing FW to send
interrupts to polling-only CQs in certain corner cases.

These two factors would allow a polling-only kernel CQ to be triggered
by an EQ interrupt and call a completion function intended only for user
CQs, causing a null pointer exception.

Some areas in the driver have prevented this issue with one-off fixes
but did not address the root cause.

This patch fixes the described issue by adding defaults to the create CQ
flow. It adds a default dummy completion function to protect against
null pointer exceptions, and it sets an invalid command sequence number
by default in kernel CQs to prevent the FW from sending an interrupt to
the CQ until it is armed. User CQs are responsible for their own
initialization values.

Callers of mlx5_core_create_cq are responsible for changing the
completion function and arming the CQ per their needs.

## Which versions are affected by CVE-2025-68209?


## How do I fix CVE-2025-68209?

Apply the vendor's patch.

## Is CVE-2025-68209 being exploited?

There is no known exploitation of CVE-2025-68209 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2025-68209 and fix it if it is affected.

What it is: CVE-2025-68209 is a vulnerability in Linux, published on 16 Dec 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-2025-68209 in the wild and no public exploit code.

Affected product: Linux. 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-2025-68209/ (Markdown: https://insights.sbomify.com/CVE-2025-68209/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-68209
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-68209
- OSV: https://osv.dev/vulnerability/CVE-2025-68209

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-68209/
Summaries are generated and may contain mistakes; the official description above is the record.