[PATCH v2] hfs/hfsplus: serialize B-tree close against folio release

[Bruno Produit <bruno.produit@trailofbits.com>]

B-tree nodes with a zero reference count remain in the node hash until
folio reclaim or tree teardown frees them.  The folio release callbacks
remove nodes while holding hash_lock, but hfs_btree_close() walks and
frees the same hash without that lock.  Reclaim can therefore unhash and
free a node after close has loaded its pointer, causing a use-after-free
or double-free. 

The following syzkaller crashes seem to be the same UAF in 
{hfs,hfsplus}_btree_close() (or {hfs,hfsplus}_bnode_unhash()) and follow 
the same pattern, but do not contain a reproducer to confirm. 

Detach each node with hfs_bnode_unhash() while holding hash_lock before
inspecting and freeing it.  Drop the lock before hfs_bnode_free() so a
large tree is not freed while holding a spinlock.  Apply the same fix to
the matching HFS+ implementation.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Reported-by: syzbot+d729df28d933979e017a@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=d729df28d933979e017a
Reported-by: syzbot+2eac7d175baf21e6a5d5@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=2eac7d175baf21e6a5d5
Reported-by: syzbot+7155b2fe09e033c91381@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=7155b2fe09e033c91381
Reported-by: syzbot+adeb387cede15eb11607@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=adeb387cede15eb11607
Reported-by: syzbot+ae7f2423f3648100506d@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=ae7f2423f3648100506d
Assisted-by: Codex:gpt-5.6-sol
Reported-by: Kyle Zeng <kylebot@openai.com>
Signed-off-by: Kyle Zeng <kylebot@openai.com>
Signed-off-by: Bruno Produit <bruno.produit@trailofbits.com>
---
 fs/hfs/btree.c     | 7 +++++--
 fs/hfsplus/btree.c | 7 +++++--
 2 files changed, 10 insertions(+), 4 deletions(-)

diff --git a/fs/hfs/btree.c b/fs/hfs/btree.c
index 41b4e8fc9..4f0ddc76e 100644
--- a/fs/hfs/btree.c
+++ b/fs/hfs/btree.c
@@ -310,14 +310,17 @@ void hfs_btree_close(struct hfs_btree *tree)
 		return;
 
 	for (i = 0; i < NODE_HASH_SIZE; i++) {
+		spin_lock(&tree->hash_lock);
 		while ((node = tree->node_hash[i])) {
-			tree->node_hash[i] = node->next_hash;
+			hfs_bnode_unhash(node);
+			spin_unlock(&tree->hash_lock);
 			if (atomic_read(&node->refcnt))
 				pr_err("node %d:%d still has %d user(s)!\n",
 				       node->tree->cnid, node->this,
 				       atomic_read(&node->refcnt));
 			hfs_bnode_free(node);
-			tree->node_hash_cnt--;
+			spin_lock(&tree->hash_lock);
 		}
+		spin_unlock(&tree->hash_lock);
 	}
 	iput(tree->inode);
diff --git a/fs/hfsplus/btree.c b/fs/hfsplus/btree.c
index 2ea8cd565..bd4dbdbc8 100644
--- a/fs/hfsplus/btree.c
+++ b/fs/hfsplus/btree.c
@@ -417,15 +417,18 @@ void hfs_btree_close(struct hfs_btree *tree)
 		return;
 
 	for (i = 0; i < NODE_HASH_SIZE; i++) {
+		spin_lock(&tree->hash_lock);
 		while ((node = tree->node_hash[i])) {
-			tree->node_hash[i] = node->next_hash;
+			hfs_bnode_unhash(node);
+			spin_unlock(&tree->hash_lock);
 			if (atomic_read(&node->refcnt))
 				pr_crit("node %d:%d "
 						"still has %d user(s)!\n",
 					node->tree->cnid, node->this,
 					atomic_read(&node->refcnt));
 			hfs_bnode_free(node);
-			tree->node_hash_cnt--;
+			spin_lock(&tree->hash_lock);
 		}
+		spin_unlock(&tree->hash_lock);
 	}
 	iput(tree->inode);