# CVE-2026-89767: ovl: fix double end_creating() on the casefold-mismatch path

> An unprivileged user can cause a double unlock and double free in overlayfs, leading to a denial of service by wedging file creation operations.

- **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-2026-89767, DEBIAN-CVE-2026-89767, GHSA-mvh9-78p9-62x7, UBUNTU-CVE-2026-89767

## What is CVE-2026-89767?

CVE-2026-89767 is a high-severity 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:

ovl: fix double end_creating() on the casefold-mismatch path

ovl_create_real() releases the new dentry twice when the casefold
consistency check fails.  The S_IFDIR branch calls end_creating() and
sets err, then falls through to the common out: label which calls
end_creating() on the same dentry again:

	case S_IFDIR:
		newdentry = ovl_do_mkdir(ofs, dir, newdentry, attr->mode);
		err = PTR_ERR_OR_ZERO(newdentry);
		if (!err && ofs->casefold != ovl_dentry_casefolded(newdentry)) {
			pr_warn_ratelimited(...);
			end_creating(newdentry);	/* first */
			err = -EINVAL;
		}
		break;
	...
	if (err)
		goto out;
	...
 out:
	if (err) {
		end_creating(newdentry);	/* second, same dentry */
		return ERR_PTR(err);
	}

end_creating() is end_dirop(), which does inode_unlock() on the parent
and dput() on the dentry, so the parent directory's i_rwsem is unlocked
twice and the dentry is put twice.  The second unlock releases a lock
that is not held, which is what wedges every later creation under that
parent, and the second dput() drops a reference that was never taken.

The branch was added by commit dfc7da402ccc ("ovl: Check for casefold
consistency when creating new dentries") as a bare dput(), which already
released the reference twice; commit fe497f0759e0 ("VFS: change
vfs_mkdir() to unlock on failure.") converted both sites to
end_creating(), adding the double unlock.

This is reachable by an unprivileged user.  The casefold consistency of
the layers is validated at mount time in ovl_parse_layer(), and again on
every lookup in ovl_lookup_single(), but ofs->workdir is the internal
"work" subdirectory created inside the user-supplied workdir, and that
subdirectory is not re-checked.  Marking it casefolded after the mount
therefore makes every ovl_create_temp() inherit the wrong state - and
that path reaches ovl_create_real() through ovl_start_creating_temp(),
which uses start_creating() with a generated name and so never runs the
lookup-time check.

  unshare -Urm
  mount -t tmpfs -o casefold=utf8-12.1.0 tmpfs mnt
  mkdir -p mnt/lower/d mnt/upper mnt/work mnt/merged
  mount -t overlay ovl -o lowerdir=mnt/lower,\
        upperdir=mnt/upper,workdir=mnt/work mnt/merged
  chattr +F mnt/work/work
  mkdir mnt/merged/d/sub		# directory copy-up

  overlayfs: wrong inherited casefold (work/#5)

and the next copy-up blocks forever on the parent's i_rwsem:

  mkdir           D  start_creating+0x65/0xb0
                     ovl_start_creating_temp+0xb0/0xe0 [overlay]
                     ovl_create_temp+0xa3/0x1d0 [overlay]
                     ovl_copy_up_one+0x1f1c/0x21c0 [overlay]
                     ovl_copy_up_flags+0xf5/0x140 [overlay]
                     ovl_create_object+0xb7/0x220 [overlay]
                     ovl_mkdir+0x23/0x40 [overlay]

Drop the end_creating() from the branch and let out: own the cleanup,
which is what every other error path in this function already does.

## Which versions are affected by CVE-2026-89767?


## How do I fix CVE-2026-89767?

Apply the vendor's patch.

## Is CVE-2026-89767 being exploited?

There is no known exploitation of CVE-2026-89767 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-89767 and fix it if it is affected.

What it is: CVE-2026-89767 is a high-severity 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-2026-89767 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-2026-89767/ (Markdown: https://insights.sbomify.com/CVE-2026-89767/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-89767
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-89767
- OSV: https://osv.dev/vulnerability/CVE-2026-89767

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-89767/
Summaries are generated and may contain mistakes; the official description above is the record.