# CVE-2026-89732: usb: gadget: f_fs: Prevent deadlock during ep0 read loop

> A local attacker can cause a kernel deadlock in the USB gadget functionfs driver by triggering a race condition during endpoint teardown, leading to system denial of service.

- **Severity:** Not scored
- **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 2026
- **Updated:** 19 Oct 2026
- **Also known as:** AZL-100160, BELL-CVE-2026-89732, DEBIAN-CVE-2026-89732, GHSA-95m6-h3qf-8pc2, UBUNTU-CVE-2026-89732

## What is CVE-2026-89732?

CVE-2026-89732 is a denial of service vulnerability in Linux, published on 11 Sep 2026. 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:

usb: gadget: f_fs: Prevent deadlock during ep0 read loop

Currently, ffs_ep0_read() holds ffs->mutex when it prepares to go to
sleep waiting for an event. When no setup events are pending, it calls
wait_event_interruptible_exclusive_locked_irq() with the mutex still
held. The wait macro deliberately drops the waitqueue spinlock before
sleeping but does not drop the mutex.

If a userspace daemon is polling ep0 via read() and the gadget is
asynchronously torn down via configfs (e.g., echo "" > UDC), a
deadlock can occur:

1. The configfs teardown calls functionfs_unbind(), which queues a
   FUNCTIONFS_UNBIND event.
2. The daemon wakes up, consumes the event, and drops the mutex.
3. However, if the daemon loops and immediately issues another read()
   before exiting, it reacquires ffs->mutex and again goes into an
   interruptible sleep.
4. Meanwhile, functionfs_unbind() continues execution and attempts to
   acquire ffs->mutex to tear down ep0req.
5. The kernel deadlocks because the configfs thread is stuck in an
   uninterruptible sleep waiting for the mutex, while the userspace
   daemon is in an interruptible sleep holding the mutex forever
   because no more events will arrive.

To fix this, we drop both the waitqueue spinlock and ffs->mutex before
going to sleep, and use wait_event_interruptible_exclusive() instead.
Upon waking up, we jump back to the `retry` label to safely reacquire
the mutex and re-evaluate the state machine. By not sleeping with
ffs->mutex held, we natively decouple gadget teardowns (which require
the mutex) from userspace polling.

## Which versions are affected by CVE-2026-89732?


## How do I fix CVE-2026-89732?

Apply the vendor's patch.

## Is CVE-2026-89732 being exploited?

There is no known exploitation of CVE-2026-89732 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-89732 and fix it if it is affected.

What it is: CVE-2026-89732 is a denial of service vulnerability in Linux, published on 11 Sep 2026. 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-2026-89732 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-2026-89732/ (Markdown: https://insights.sbomify.com/CVE-2026-89732/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-89732
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-89732
- OSV: https://osv.dev/vulnerability/CVE-2026-89732

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-89732/
Summaries are generated and may contain mistakes; the official description above is the record.