CVE-2026-89560

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

landlock: Require LANDLOCK_ACCESS_FS_MAKE_REG for whiteout creation

Whiteout objects are used in the upper layer of an OverlayFS to

indicate that the file with this name does not exist in the unified

view, even if it is present in one of the lower layer file systems.

For the userspace implementations of OverlayFS (fuse-overlayfs),

whiteout objects can be created from userspace as well:

* mknod(2) with S_IFCHR and makedev(0, 0)

* renameat2(2) with RENAME_WHITEOUT,

creating the whiteout in the old place of the moved file.

This commit guards whiteout creation in both of these cases with

LANDLOCK_ACCESS_FS_MAKE_REG. Whiteout objects are *not* considered

character devices and are not bound to a driver.

LANDLOCK_ACCESS_FS_MAKE_REG describes the same permission class as a

whiteout object: creating one is the only S_IFCHR creation that the VFS

exempts from CAP_MKNOD, so it is as unprivileged as creating a regular

file, while LANDLOCK_ACCESS_FS_MAKE_CHAR and

LANDLOCK_ACCESS_FS_MAKE_BLOCK keep meaning the creation of devices that

expose a kernel interface [1].

For the mknod(2) case, introduce a Landlock erratum. The creation of

whiteout objects through mknod(2) was previously guarded using

LANDLOCK_ACCESS_FS_MAKE_CHAR, and it is now guarded using

LANDLOCK_ACCESS_FS_MAKE_REG.

For the renameat2(2) case, fix a bug: Before this commit, renameat2(2)

with RENAME_WHITEOUT would create a directory entry even when all

LANDLOCK_ACCESS_FS_MAKE_* rights were denied.

This does not affect normal renames within layered OverlayFS mounts:

When doing a regular rename() on a mounted fuse-overlayfs, it is the

fuse-overlayfs daemon that exercises renameat2() with RENAME_WHITEOUT,

and only the Landlock domain of that daemon is checked there.

Depends-on: 49c9e09d9610 ("landlock: Fix handling of disconnected directories")

Depends-on: fe72ce6710cb ("landlock: Add errata documentation section")

[mic: Record why LANDLOCK_ACCESS_FS_MAKE_REG is the matching right, and

add link(2) to the user doc]