[PATCH 6.6 241/538] nilfs2: fix potential oob read in nilfs_btree_check_delete()

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6.6-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Ryusuke Konishi <konishi.ryusuke@xxxxxxxxx>

[ Upstream commit f9c96351aa6718b42a9f42eaf7adce0356bdb5e8 ]

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.

Link: https://lkml.kernel.org/r/20240904081401.16682-4-konishi.ryusuke@xxxxxxxxx
Fixes: 17c76b0104e4 ("nilfs2: B-tree based block mapping")
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@xxxxxxxxx>
Cc: Lizhi Xu <lizhi.xu@xxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 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 dac090c162a7a..dbd27a44632fa 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







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