CVE-2026-89560: landlock: Require LANDLOCK_ACCESS_FS_MAKE_REG for whiteout creation

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

From: Greg Kroah-Hartman <gregkh@kernel.org>

Description
===========

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]

The Linux kernel CVE team has assigned CVE-2026-89560 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 5.13 with commit cb2c7d1a1776057c9a1f48ed1250d85e94d4850d and fixed in 6.18.50 with commit 627ce4902df1d737e99306daae5a87c68b876d16
	Issue introduced in 5.13 with commit cb2c7d1a1776057c9a1f48ed1250d85e94d4850d and fixed in 7.2.4 with commit 0c3204aacbe8c07f9e87e3028f28ec6c166a1075
	Issue introduced in 5.13 with commit cb2c7d1a1776057c9a1f48ed1250d85e94d4850d and fixed in 7.3-rc1 with commit 672fa082d48b21e1fb62cdb184fee41513e53421

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-89560
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	include/uapi/linux/landlock.h
	security/landlock/errata/abi-1.h
	security/landlock/fs.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/627ce4902df1d737e99306daae5a87c68b876d16
	https://git.kernel.org/stable/c/0c3204aacbe8c07f9e87e3028f28ec6c166a1075
	https://git.kernel.org/stable/c/672fa082d48b21e1fb62cdb184fee41513e53421