# CVE-2026-89670: nfsd: hold rcu across localio cmpxchg retry

> A race condition in the Linux kernel NFS server allows attackers to trigger a use-after-free, potentially leading to code execution or privilege escalation.

- **Severity:** High 7.8 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/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 2026
- **Updated:** 19 Oct 2026
- **Also known as:** BELL-CVE-2026-89670, DEBIAN-CVE-2026-89670, GHSA-86jr-9j73-5pc8, UBUNTU-CVE-2026-89670

## What is CVE-2026-89670?

CVE-2026-89670 is a high-severity use after free 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:

nfsd: hold rcu across localio cmpxchg retry

nfsd_file objects are freed via call_rcu (filecache.c:296), and
nfsd_file_slab is created without SLAB_TYPESAFE_BY_RCU
(KMEM_CACHE(nfsd_file, 0) at filecache.c:789), so the slab page
backing a freed nfsd_file becomes freely reclaimable once the RCU
grace period elapses.

The again: retry block in nfsd_open_local_fh() loads a pointer with
cmpxchg and then calls nfsd_file_get(new) (which is
refcount_inc_not_zero) without holding rcu_read_lock. The sole caller
nfs_open_local_fh() drops rcu_read_lock before invoking this helper,
so no outer reader-side critical section covers the load.

    CPU 0 (nfsd_open_local_fh)        CPU 1 (nfsd_file_put_local)
    -----                             -----
    new = cmpxchg(pnf, NULL, ...)
                                      nf = xchg(pnf, NULL)
                                      nfsd_file_put(nf)
                                        last ref -> call_rcu()
                                      /* grace period elapses;
                                         slab page recycled */
    nfsd_file_get(new)
      refcount_inc_not_zero(&new->nf_ref)
      /* operates on recycled memory */

A non-zero word at the nf_ref offset of the recycled object makes the
refcount bump appear to succeed, and the caller then dereferences
new->nf_net and new->nf_file out of freed memory.

Fix by taking rcu_read_lock() immediately before the cmpxchg and
releasing it on all three exits of the if (new) block: the goto-again
retry, the lost-race cleanup path, and the install-succeeded path.
nfsd_file_put() and nfsd_net_put() stay outside the RCU section so
they remain free to block.

## Which versions are affected by CVE-2026-89670?


## How do I fix CVE-2026-89670?

Apply the vendor's patch.

## Is CVE-2026-89670 being exploited?

There is no known exploitation of CVE-2026-89670 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-89670 and fix it if it is affected.

What it is: CVE-2026-89670 is a high-severity use after free 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-2026-89670 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-2026-89670/ (Markdown: https://insights.sbomify.com/CVE-2026-89670/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-89670
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-89670
- OSV: https://osv.dev/vulnerability/CVE-2026-89670

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-89670/
Summaries are generated and may contain mistakes; the official description above is the record.