CVE-2026-74343: kernfs: fix xattr race condition with multiple superblocks

CVE-2026-74343: kernfs: fix xattr race condition with multiple superblocks

[Greg Kroah-Hartman]

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

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

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

kernfs: fix xattr race condition with multiple superblocks

Multiple superblocks with different namespaces can share the same
kernfs_node when kernfs_test_super() finds a matching root but
different namespace. This means multiple inodes from different
superblocks can reference the same kernfs_node->iattr->xattrs
structure.

The VFS layer only holds per-inode locks during xattr operations,
which is insufficient to serialize concurrent xattr modifications on
the shared kernfs_node. This can lead to race conditions in
simple_xattr_set() where the lookup->replace/remove sequence is not
atomic with respect to operations from other superblocks.

Fix this by protecting xattr operations with the existing hashed
kernfs_locks->open_file_mutex[] array, which is already used to
protect per-node open file data. The hashed mutex array provides
scalable per-node serialization (scaled by CPU count, up to 1024 locks
on 32+ CPU systems) with zero memory overhead.

Changes:
- Rename open_file_mutex[] to node_mutex[] to reflect dual purpose
- Add kernfs_node_lock_ptr() and kernfs_node_lock() helpers
- Protect simple_xattr_set() calls in kernfs_xattr_set() and
  kernfs_vfs_user_xattr_set() with the hashed mutex
- Update file.c to use new helpers via compatibility wrappers
- Update documentation to explain the extended lock usage

The Linux kernel CVE team has assigned CVE-2026-74343 to this issue.


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

	Issue introduced in 7.1 with commit b32c4a213698ab351b44da2fd1b2a5976c7fa033 and fixed in 7.1.5 with commit bf53db51359939755084d08156684ed649489592
	Issue introduced in 7.1 with commit b32c4a213698ab351b44da2fd1b2a5976c7fa033 and fixed in 7.2-rc1 with commit 6a07814ff643b5c8e1353d8c6229f52fde205cde

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-2026-74343
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:
	fs/kernfs/file.c
	fs/kernfs/inode.c
	fs/kernfs/kernfs-internal.h
	fs/kernfs/mount.c
	include/linux/kernfs.h


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/bf53db51359939755084d08156684ed649489592
	https://git.kernel.org/stable/c/6a07814ff643b5c8e1353d8c6229f52fde205cde