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

Fixes: b32c4a213698 ("xattr: add rhashtable-based simple_xattr infrastructure")
Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://sashiko.dev/#/patchset/20260601162454.2116375-1-mszeredi%40redhat.com
Assisted-by: Claude:claude-sonnet-4-5
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Link: https://patch.msgid.link/20260605135322.2632068-2-mszeredi@redhat.com
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

1 files changed, 8 insertions, 3 deletions

diff --git a/include/linux/kernfs.h b/include/linux/kernfs.h
index e21b2f7f4159..351a5101c862 100644
--- a/include/linux/kernfs.h
+++ b/include/linux/kernfs.h
@@ -76,20 +76,25 @@ struct kernfs_iattrs;
  * kernfs_open_file.
  *
  * kernfs_open_files are chained at kernfs_open_node->files, which is
- * protected by kernfs_global_locks.open_file_mutex[i].
+ * protected by kernfs_global_locks.node_mutex[i].
  *
  * To reduce possible contention in sysfs access, arising due to single
- * locks, use an array of locks (e.g. open_file_mutex) and use kernfs_node
+ * locks, use an array of locks (e.g. node_mutex) and use kernfs_node
  * object address as hash keys to get the index of these locks.
  *
  * Hashed mutexes are safe to use here because operations using these don't
  * rely on global exclusion.
  *
+ * The hashed mutex array protects per-node data: the kernfs_open_node for
+ * open file management, and kernfs_node xattr operations (necessary because
+ * multiple superblocks with different namespaces can share the same
+ * kernfs_node, making per-inode locking insufficient).
+ *
  * In future we intend to replace other global locks with hashed ones as well.
  * kernfs_global_locks acts as a holder for all such hash tables.
  */
 struct kernfs_global_locks {
-	struct mutex open_file_mutex[NR_KERNFS_LOCKS];
+	struct mutex node_mutex[NR_KERNFS_LOCKS];
 };
 
 enum kernfs_node_type {