Linux Kernel CVE-2025-68774

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

hfsplus: fix missing hfs_bnode_get() in __hfs_bnode_create

When sync() and link() are called concurrently, both threads may enter hfs_bnode_find() without finding the node in the hash table and proceed to create it.

Thread A: hfsplus_write_inode() -> hfsplus_write_system_inode() -> hfs_btree_write() -> hfs_bnode_find(tree, 0) -> __hfs_bnode_create(tree, 0)

Thread B: hfsplus_create_cat() -> hfs_brec_insert() -> hfs_bnode_split() -> hfs_bmap_alloc() -> hfs_bnode_find(tree, 0) -> __hfs_bnode_create(tree, 0)

In this case, thread A creates the bnode, sets refcnt=1, and hashes it. Thread B also tries to create the same bnode, notices it has already been inserted, drops its own instance, and uses the hashed one without getting the node.

	node2 = hfs_bnode_findhash(tree, cnid);
	if (!node2) {                                 <- Thread A
		hash = hfs_bnode_hash(cnid);
		node->next_hash = tree->node_hash[hash];
		tree->node_hash[hash] = node;
		tree->node_hash_cnt++;
	} else {                                      <- Thread B
		spin_unlock(&tree->hash_lock);
		kfree(node);
		wait_event(node2->lock_wq,
			!test_bit(HFS_BNODE_NEW, &node2->flags));
		return node2;
	}

However, hfs_bnode_find() requires each call to take a reference. Here both threads end up setting refcnt=1. When they later put the node, this triggers:

BUG_ON(!atomic_read(&node->refcnt))

In this scenario, Thread B in fact finds the node in the hash table rather than creating a new one, and thus must take a reference.

Fix this by calling hfs_bnode_get() when reusing a bnode newly created by another thread to ensure the refcount is updated correctly.

A similar bug was fixed in HFS long ago in commit a9dc087fd3c4 ("fix missing hfs_bnode_get() in __hfs_bnode_create") but the same issue remained in HFS+ until now.

The affected product is the mainline Linux kernel's hfsplus filesystem driver; the flaw exists in all versions prior to the fix and mirrors an issue long resolved in the older HFS driver. NVD references seven git.kernel.org stable-tree commits representing the fix backported across supported stable branches (e.g., https://git.kernel.org/stable/c/b9d1c6bb5f19460074ce9862cb80be86b5fb0a50 and https://git.kernel.org/stable/c/152af114287851583cf7e0abc10129941f19466a). Distribution exposure is confirmed for Ubuntu via advisory USN-8096-1 (https://ubuntu.com/security/notices/USN-8096-1). Exact vulnerable-to-fixed version boundaries per branch are not enumerated in the provided data and should be confirmed against each distribution's kernel package and the specific stable commit that landed in that branch.