fs: fix user path of nested backing files

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Summary

CVECVE-2026-89768
StatePUBLISHED
AssignerLinux
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-09-11 20:20:08 UTC
Updated2026-09-11 20:20:08 UTC
DescriptionIn 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.

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Linux Linux affected 5b6aa9a843205da92d860e5011a7b29062a76b8f 88c927a63dc717b6d46b20fe13ea713916e49089 git Not specified
CNA Linux Linux affected 5dfcb15974e7d0f96aca278dd9f1b85df91523ef c03114634d342648bd34910aa8fb88007e92cc3c git Not specified
CNA Linux Linux affected 6af36aeb147a06dea47c49859cd6ca5659aeb987 a35cc21355734e1acb89973d9be90ca8e4c3ed2f git Not specified
CNA Linux Linux affected 6af36aeb147a06dea47c49859cd6ca5659aeb987 f2381b546e7e6a35c9fcee0d0ccb6c042a9aeb5d git Not specified
CNA Linux Linux affected 41c5b269af8b1f0bffcab7766a793f294ae6764e git Not specified
CNA Linux Linux affected 27e795afba0018b0ea9460dbad4bd706d1ba5ee0 git Not specified
CNA Linux Linux affected 6.12.95 6.12.109 semver Not specified
CNA Linux Linux affected 6.18.38 6.18.50 semver Not specified
CNA Linux Linux affected 6.6.144 6.7 semver Not specified
CNA Linux Linux affected 7.0.4 7.1 semver Not specified
CNA Linux Linux affected 7.1 Not specified
CNA Linux Linux unaffected 7.1 semver Not specified
CNA Linux Linux unaffected 6.12.109 6.12.* semver Not specified
CNA Linux Linux unaffected 6.18.50 6.18.* semver Not specified
CNA Linux Linux unaffected 7.2.4 7.2.* semver Not specified
CNA Linux Linux unaffected 7.3-rc1 * original_commit_for_fix Not specified

References

ReferenceSourceLinkTags
git.kernel.org/stable/c/f2381b546e7e6a35c9fcee0d0ccb6c042a9aeb5d 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org
git.kernel.org/stable/c/88c927a63dc717b6d46b20fe13ea713916e49089 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org
git.kernel.org/stable/c/a35cc21355734e1acb89973d9be90ca8e4c3ed2f 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org
git.kernel.org/stable/c/c03114634d342648bd34910aa8fb88007e92cc3c 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org
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