# CVE-2025-40220: fuse: fix livelock in synchronous file put from fuseblk workers

> A local attacker can cause a system hang in the Linux kernel FUSE module by triggering a livelock during synchronous file operations.

- **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:** 4 Dec 2025
- **Updated:** 23 May 2026
- **Also known as:** AZL-71290, BELL-CVE-2025-40220, DEBIAN-CVE-2025-40220, GHSA-f6j8-wgmp-w2x6, ROOT-OS-DEBIAN-11-CVE-2025-40220, ROOT-OS-UBUNTU-2404-CVE-2025-40220, UBUNTU-CVE-2025-40220

## What is CVE-2025-40220?

CVE-2025-40220 is a denial of service vulnerability in Linux, published on 4 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:

fuse: fix livelock in synchronous file put from fuseblk workers

I observed a hang when running generic/323 against a fuseblk server.
This test opens a file, initiates a lot of AIO writes to that file
descriptor, and closes the file descriptor before the writes complete.
Unsurprisingly, the AIO exerciser threads are mostly stuck waiting for
responses from the fuseblk server:

# cat /proc/372265/task/372313/stack
[<0>] request_wait_answer+0x1fe/0x2a0 [fuse]
[<0>] __fuse_simple_request+0xd3/0x2b0 [fuse]
[<0>] fuse_do_getattr+0xfc/0x1f0 [fuse]
[<0>] fuse_file_read_iter+0xbe/0x1c0 [fuse]
[<0>] aio_read+0x130/0x1e0
[<0>] io_submit_one+0x542/0x860
[<0>] __x64_sys_io_submit+0x98/0x1a0
[<0>] do_syscall_64+0x37/0xf0
[<0>] entry_SYSCALL_64_after_hwframe+0x4b/0x53

But the /weird/ part is that the fuseblk server threads are waiting for
responses from itself:

# cat /proc/372210/task/372232/stack
[<0>] request_wait_answer+0x1fe/0x2a0 [fuse]
[<0>] __fuse_simple_request+0xd3/0x2b0 [fuse]
[<0>] fuse_file_put+0x9a/0xd0 [fuse]
[<0>] fuse_release+0x36/0x50 [fuse]
[<0>] __fput+0xec/0x2b0
[<0>] task_work_run+0x55/0x90
[<0>] syscall_exit_to_user_mode+0xe9/0x100
[<0>] do_syscall_64+0x43/0xf0
[<0>] entry_SYSCALL_64_after_hwframe+0x4b/0x53

The fuseblk server is fuse2fs so there's nothing all that exciting in
the server itself.  So why is the fuse server calling fuse_file_put?
The commit message for the fstest sheds some light on that:

"By closing the file descriptor before calling io_destroy, you pretty
much guarantee that the last put on the ioctx will be done in interrupt
context (during I/O completion).

Aha.  AIO fgets a new struct file from the fd when it queues the ioctx.
The completion of the FUSE_WRITE command from userspace causes the fuse
server to call the AIO completion function.  The completion puts the
struct file, queuing a delayed fput to the fuse server task.  When the
fuse server task returns to userspace, it has to run the delayed fput,
which in the case of a fuseblk server, it does synchronously.

Sending the FUSE_RELEASE command sychronously from fuse server threads
is a bad idea because a client program can initiate enough simultaneous
AIOs such that all the fuse server threads end up in delayed_fput, and
now there aren't any threads left to handle the queued fuse commands.

Fix this by only using asynchronous fputs when closing files, and leave
a comment explaining why.

## Which versions are affected by CVE-2025-40220?


## How do I fix CVE-2025-40220?

Apply the vendor's patch.

## Is CVE-2025-40220 being exploited?

There is no known exploitation of CVE-2025-40220 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2025-40220 and fix it if it is affected.

What it is: CVE-2025-40220 is a denial of service vulnerability in Linux, published on 4 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-2025-40220 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-2025-40220/ (Markdown: https://insights.sbomify.com/CVE-2025-40220/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-40220
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-40220
- OSV: https://osv.dev/vulnerability/CVE-2025-40220

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-40220/
Summaries are generated and may contain mistakes; the official description above is the record.