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.
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.
Analysis
No analysis available yet.
Default status is the baseline for the product, each version can override it (e.g. patched versions marked unaffected).
| Vendor | Product | Default status | Versions | |||||||||||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Linux | Linux | unaffected |
|
|||||||||||||||||||||||||||||||||
| Linux | Linux | affected |
|
No data.
No data.
No data available yet.
Remediation
No remediation available yet.
Tracking
Sign in to view the affected projects.
Advisories
No advisories yet.
References
History
Sat, 12 Sep 2026 00:15:00 +0000
| Type | Values Removed | Values Added |
|---|---|---|
| Weaknesses | CWE-41 | |
| References |
| |
| Metrics |
threat_severity
|
cvssV3_1
|
Fri, 11 Sep 2026 23:45:00 +0000
| Type | Values Removed | Values Added |
|---|---|---|
| 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. | |
| Title | fs: fix user path of nested backing files | |
| First Time appeared |
Linux
Linux linux Kernel |
|
| CPEs | cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:* | |
| Vendors & Products |
Linux
Linux linux Kernel |
|
| References |
|
MITRE
Status: PUBLISHED
Assigner: Linux
Published:
Updated: 2026-09-11T19:47:08.119Z
Reserved: 2026-09-11T19:38:34.764Z
Link: CVE-2026-89768
Vulnrichment
No data.
NVD
Status : Received
Published: 2026-09-11T20:20:08.090
Modified: 2026-09-11T20:20:08.090
Link: CVE-2026-89768
Redhat
OpenCVE Enrichment
No data.
Weaknesses
-
CWE-41
Improper Resolution of Path Equivalence