CVE-2025-68774

Summary

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.

Affected Software

Weaknesses

References

• https://git.kernel.org/stable/c/3b0fc7af50b896d0f3d104e70787ba1973bc0b56
• https://git.kernel.org/stable/c/39e149d58ef4d7883cbf87448d39d51292fd342d
• https://git.kernel.org/stable/c/b68dc4134b18a3922cd33439ec614aad4172bc86
• https://git.kernel.org/stable/c/b9d1c6bb5f19460074ce9862cb80be86b5fb0a50
• https://git.kernel.org/stable/c/457f795e7abd7770de10216d7f9994a3f12a56d6
• https://git.kernel.org/stable/c/5882e7c8cdbb5e254a69628b780acff89c78071e
• https://git.kernel.org/stable/c/152af114287851583cf7e0abc10129941f19466a

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