AZL-74445

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

hfsplus: fix missing hfsbnodeget() in __hfsbnodecreate

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

Thread A: hfspluswriteinode() -> hfspluswritesysteminode() -> hfsbtreewrite() -> hfsbnode_find(tree, 0) -> __hfsbnodecreate(tree, 0)

Thread B: hfspluscreatecat() -> hfsbrecinsert() -> hfsbnodesplit() -> hfsbmapalloc() -> hfsbnodefind(tree, 0) -> __hfsbnodecreate(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, hfsbnodefind() requires each call to take a reference. Here both threads end up setting refcnt=1. When they later put the node, this triggers:

BUGON(!atomicread(&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 hfsbnodeget() 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 hfsbnodeget() in __hfsbnodecreate") but the same issue remained in HFS+ until now.