[PATCH 3/3] nilfs2: fix potential oob read in nilfs_btree_check_delete()

[Ryusuke Konishi <konishi.ryusuke@gmail.com>]

The function nilfs_btree_check_delete(), which checks whether
degeneration to direct mapping occurs before deleting a b-tree entry,
causes memory access outside the block buffer when retrieving the
maximum key if the root node has no entries.

This does not usually happen because b-tree mappings with 0 child
nodes are never created by mkfs.nilfs2 or nilfs2 itself.  However, it
can happen if the b-tree root node read from a device is configured
that way, so fix this potential issue by adding a check for that case.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Fixes: 17c76b0104e4 ("nilfs2: B-tree based block mapping")
---
 fs/nilfs2/btree.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/fs/nilfs2/btree.c b/fs/nilfs2/btree.c
index dedd3c480842..ef5061bb56da 100644
--- a/fs/nilfs2/btree.c
+++ b/fs/nilfs2/btree.c
@@ -1659,13 +1659,16 @@ static int nilfs_btree_check_delete(struct nilfs_bmap *btree, __u64 key)
 	int nchildren, ret;
 
 	root = nilfs_btree_get_root(btree);
+	nchildren = nilfs_btree_node_get_nchildren(root);
+	if (unlikely(nchildren == 0))
+		return 0;
+
 	switch (nilfs_btree_height(btree)) {
 	case 2:
 		bh = NULL;
 		node = root;
 		break;
 	case 3:
-		nchildren = nilfs_btree_node_get_nchildren(root);
 		if (nchildren > 1)
 			return 0;
 		ptr = nilfs_btree_node_get_ptr(root, nchildren - 1,
@@ -1674,12 +1677,12 @@ static int nilfs_btree_check_delete(struct nilfs_bmap *btree, __u64 key)
 		if (ret < 0)
 			return ret;
 		node = (struct nilfs_btree_node *)bh->b_data;
+		nchildren = nilfs_btree_node_get_nchildren(node);
 		break;
 	default:
 		return 0;
 	}
 
-	nchildren = nilfs_btree_node_get_nchildren(node);
 	maxkey = nilfs_btree_node_get_key(node, nchildren - 1);
 	nextmaxkey = (nchildren > 1) ?
 		nilfs_btree_node_get_key(node, nchildren - 2) : 0;
-- 
2.43.0