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//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 Software

References

• https://git.kernel.org/stable/c/88c927a63dc717b6d46b20fe13ea713916e49089
• https://git.kernel.org/stable/c/a35cc21355734e1acb89973d9be90ca8e4c3ed2f
• https://git.kernel.org/stable/c/c03114634d342648bd34910aa8fb88007e92cc3c
• https://git.kernel.org/stable/c/f2381b546e7e6a35c9fcee0d0ccb6c042a9aeb5d