# CVE-2026-89768

## Summary

- **CVE ID:** CVE-2026-89768
- **Severity:** UNKNOWN
- **CVSS Score:** 1.51
- **CWE:** N/A
- **Published:** Sep 11, 2026
- **Last Modified:** Sep 16, 2026

## 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.

## Affected Products

- Linux — Linux (5b6aa9a843205da92d860e5011a7b29062a76b8f)
- Linux — Linux (5dfcb15974e7d0f96aca278dd9f1b85df91523ef)
- Linux — Linux (6af36aeb147a06dea47c49859cd6ca5659aeb987)
- Linux — Linux (41c5b269af8b1f0bffcab7766a793f294ae6764e)
- Linux — Linux (27e795afba0018b0ea9460dbad4bd706d1ba5ee0)
- Linux — Linux (6.12.95)
- Linux — Linux (6.18.38)
- Linux — Linux (6.6.144)
- Linux — Linux (7.0.4)
- Linux — Linux (7.1)
- Linux — Linux (0)
- Linux — Linux (6.12.109)
- Linux — Linux (6.18.50)
- Linux — Linux (7.2.4)
- Linux — Linux (7.3-rc1)

## References

- [CNA](https://git.kernel.org/stable/c/88c927a63dc717b6d46b20fe13ea713916e49089)
- [CNA](https://git.kernel.org/stable/c/c03114634d342648bd34910aa8fb88007e92cc3c)
- [CNA](https://git.kernel.org/stable/c/a35cc21355734e1acb89973d9be90ca8e4c3ed2f)
- [CNA](https://git.kernel.org/stable/c/f2381b546e7e6a35c9fcee0d0ccb6c042a9aeb5d)

## Exploitation Prediction (EPSS)

- **EPSS Score:** 0.19%
- **EPSS Percentile:** 8.5

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_Exported from OnDuty AI Vulnerability Intelligence on 2026-09-18._