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, 3 insertions, 10 deletions

diff --git a/fs/kernfs/file.c b/fs/kernfs/file.c
index 1163aa769738..8e0e90c93372 100644
--- a/fs/kernfs/file.c
+++ b/fs/kernfs/file.c
@@ -40,22 +40,15 @@ struct kernfs_open_node {
 static DEFINE_SPINLOCK(kernfs_notify_lock);
 static struct kernfs_node *kernfs_notify_list = KERNFS_NOTIFY_EOL;
 
+/* Compatibility wrappers - use the common hashed node lock */
 static inline struct mutex *kernfs_open_file_mutex_ptr(struct kernfs_node *kn)
 {
-	int idx = hash_ptr(kn, NR_KERNFS_LOCK_BITS);
-
-	return &kernfs_locks->open_file_mutex[idx];
+	return kernfs_node_lock_ptr(kn);
 }
 
 static inline struct mutex *kernfs_open_file_mutex_lock(struct kernfs_node *kn)
 {
-	struct mutex *lock;
-
-	lock = kernfs_open_file_mutex_ptr(kn);
-
-	mutex_lock(lock);
-
-	return lock;
+	return kernfs_node_lock(kn);
 }
 
 /**