# CVE-2026-89768: fs: fix user path of nested backing files

> Local attackers can read incorrect file paths from process maps in nested overlayfs configurations, potentially leaking internal filesystem structure.

- **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-100442, BELL-CVE-2026-89768, DEBIAN-CVE-2026-89768, GHSA-xv39-4768-vprw, UBUNTU-CVE-2026-89768

## What is CVE-2026-89768?

CVE-2026-89768 is a 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:

fs: fix user path of nested backing files

backing_file_open() derives the path to be stored in the new backing
file from user_file->f_path.  This is incorrect when user_file itself
is a backing file, which is the case for nested stacking filesystems,
e.g. overlayfs mounts where the lowerdir of one overlayfs is the merged
directory of another.  Since commit def3ae83da02 ("fs: store real path
instead of fake path in backing file f_path") the f_path of a backing
file holds the real path of the intermediate layer, not the path that
the user opened.

Commit 924577e4f6ca ("ovl: Fix nested backing file paths") fixed this
for such configurations by passing file_user_path() from
ovl_open_realfile().  However, commit 6af36aeb147a ("lsm: add
backing_file LSM hooks") changed the first argument of
backing_file_open() from the user path back to the user file and
derived the path from user_file->f_path again, silently re-introducing
the problem.

As a result, files mapped through a nested overlayfs show the wrong
path in /proc/<pid>/maps and in perf/ftrace mmap records.  For example,
with two nested overlayfs mounts:

  mkdir -p /ovl/{lower,upper,work,merged} /ovl/nested
  echo hello > /ovl/lower/foo
  mount -t overlay overlay \
	-o lowerdir=/ovl/lower,upperdir=/ovl/upper,workdir=/ovl/work \
	/ovl/merged
  # at least two lowerdirs are needed when upperdir is nonexistent
  mount -t overlay overlay \
	-o lowerdir=/ovl/merged:/ovl/lower /ovl/nested

mapping /ovl/nested/foo shows a disconnected path instead of the user
path:

  # readlink /proc/self/fd/3
  /ovl/nested/foo
  # grep foo /proc/self/maps
  7f6e2c100000-7f6e2c101000 r--s 00000000 00:24 15813027 /foo

The bogus path is derived from the f_path of the intermediate backing
file, whose mount is a private clone that d_path() cannot resolve.

Fix this by using file_user_path(), which returns the outermost
user-visible path for backing files and falls back to
&user_file->f_path for regular files.  This restores the behavior of
commit 924577e4f6ca ("ovl: Fix nested backing file paths") for
overlayfs and also fixes the same problem for the other
backing_file_open() callers, fuse passthrough and erofs ishare, when
their user file is itself a backing file.

backing_tmpfile_open() has the same pattern but is not affected: it is
only called by ovl_create_tmpfile() for the upper layer, and another
overlayfs is rejected as upperdir by the DCACHE_OP_REAL check in
ovl_mount_dir_check(), so its user_file can never be a backing file.

## Which versions are affected by CVE-2026-89768?


## How do I fix CVE-2026-89768?

Apply the vendor's patch.

## Is CVE-2026-89768 being exploited?

There is no known exploitation of CVE-2026-89768 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-89768 and fix it if it is affected.

What it is: CVE-2026-89768 is a 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-2026-89768 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-2026-89768/ (Markdown: https://insights.sbomify.com/CVE-2026-89768/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-89768
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-89768
- OSV: https://osv.dev/vulnerability/CVE-2026-89768

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-89768/
Summaries are generated and may contain mistakes; the official description above is the record.