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From:   Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
To:     gustavoars@kernel.org, viro@zeniv.linux.org.uk
Cc:     Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        skhan@linuxfoundation.org, gregkh@linuxfoundation.org,
        linux-kernel-mentees@lists.linuxfoundation.org,
        syzbot+b718ec84a87b7e73ade4@syzkaller.appspotmail.com
Subject: [PATCH 3/3] hfs: add lock nesting notation to hfs_find_init
Date:   Tue, 29 Jun 2021 22:48:03 +0800
Message-Id: <20210629144803.62541-4-desmondcheongzx@gmail.com>
In-Reply-To: <20210629144803.62541-1-desmondcheongzx@gmail.com>
References: <20210629144803.62541-1-desmondcheongzx@gmail.com>
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Syzbot reports a possible recursive lock:
https://syzkaller.appspot.com/bug?id=f007ef1d7a31a469e3be7aeb0fde0769b18585db

This happens due to missing lock nesting information. From the logs,
we see that a call to hfs_fill_super is made to mount the hfs
filesystem. While searching for the root inode, the lock on the
catalog btree is grabbed. Then, when the parent of the root isn't
found, a call to __hfs_bnode_create is made to create the parent of
the root. This eventually leads to a call to hfs_ext_read_extent which
grabs a lock on the extents btree.

Since the order of locking is catalog btree -> extents btree, this
lock hierarchy does not lead to a deadlock.

To tell lockdep that this locking is safe, we add nesting notation to
distinguish between catalog btrees, extents btrees, and attributes
btrees (for HFS+). This has already been done in hfsplus.

Reported-and-tested-by: syzbot+b718ec84a87b7e73ade4@syzkaller.appspotmail.com
Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
---
 fs/hfs/bfind.c | 14 +++++++++++++-
 fs/hfs/btree.h |  7 +++++++
 2 files changed, 20 insertions(+), 1 deletion(-)

diff --git a/fs/hfs/bfind.c b/fs/hfs/bfind.c
index 4af318fbda77..ef9498a6e88a 100644
--- a/fs/hfs/bfind.c
+++ b/fs/hfs/bfind.c
@@ -25,7 +25,19 @@ int hfs_find_init(struct hfs_btree *tree, struct hfs_find_data *fd)
 	fd->key = ptr + tree->max_key_len + 2;
 	hfs_dbg(BNODE_REFS, "find_init: %d (%p)\n",
 		tree->cnid, __builtin_return_address(0));
-	mutex_lock(&tree->tree_lock);
+	switch (tree->cnid) {
+	case HFS_CAT_CNID:
+		mutex_lock_nested(&tree->tree_lock, CATALOG_BTREE_MUTEX);
+		break;
+	case HFS_EXT_CNID:
+		mutex_lock_nested(&tree->tree_lock, EXTENTS_BTREE_MUTEX);
+		break;
+	case HFS_ATTR_CNID:
+		mutex_lock_nested(&tree->tree_lock, ATTR_BTREE_MUTEX);
+		break;
+	default:
+		return -EINVAL;
+	}
 	return 0;
 }
 
diff --git a/fs/hfs/btree.h b/fs/hfs/btree.h
index 4ba45caf5939..0e6baee93245 100644
--- a/fs/hfs/btree.h
+++ b/fs/hfs/btree.h
@@ -13,6 +13,13 @@ typedef int (*btree_keycmp)(const btree_key *, const btree_key *);
 
 #define NODE_HASH_SIZE  256
 
+/* B-tree mutex nested subclasses */
+enum hfs_btree_mutex_classes {
+	CATALOG_BTREE_MUTEX,
+	EXTENTS_BTREE_MUTEX,
+	ATTR_BTREE_MUTEX,
+};
+
 /* A HFS BTree held in memory */
 struct hfs_btree {
 	struct super_block *sb;
-- 
2.25.1