CVE-2025-68736

In the Linux kernel, the following vulnerability has been resolved:

landlock: Fix handling of disconnected directories

Disconnected files or directories can appear when they are visible and

opened from a bind mount, but have been renamed or moved from the source

of the bind mount in a way that makes them inaccessible from the mount

point (i.e. out of scope).

Previously, access rights tied to files or directories opened through a

disconnected directory were collected by walking the related hierarchy

down to the root of the filesystem, without taking into account the

mount point because it couldn't be found. This could lead to

inconsistent access results, potential access right widening, and

hard-to-debug renames, especially since such paths cannot be printed.

For a sandboxed task to create a disconnected directory, it needs to

have write access (i.e. FS_MAKE_REG, FS_REMOVE_FILE, and FS_REFER) to

the underlying source of the bind mount, and read access to the related

mount point. Because a sandboxed task cannot acquire more access

rights than those defined by its Landlock domain, this could lead to

inconsistent access rights due to missing permissions that should be

inherited from the mount point hierarchy, while inheriting permissions

from the filesystem hierarchy hidden by this mount point instead.

Landlock now handles files and directories opened from disconnected

directories by taking into account the filesystem hierarchy when the

mount point is not found in the hierarchy walk, and also always taking

into account the mount point from which these disconnected directories

were opened. This ensures that a rename is not allowed if it would

widen access rights [1].

The rationale is that, even if disconnected hierarchies might not be

visible or accessible to a sandboxed task, relying on the collected

access rights from them improves the guarantee that access rights will

not be widened during a rename because of the access right comparison

between the source and the destination (see LANDLOCK_ACCESS_FS_REFER).

It may look like this would grant more access on disconnected files and

directories, but the security policies are always enforced for all the

evaluated hierarchies. This new behavior should be less surprising to

users and safer from an access control perspective.

Remove a wrong WARN_ON_ONCE() canary in collect_domain_accesses() and

fix the related comment.

Because opened files have their access rights stored in the related file

security properties, there is no impact for disconnected or unlinked

files.