[PATCH v3] nilfs2: enhance btree node keys check

syzbot reported a warning on nilfs_btree_assign:
WARNING: fs/nilfs2/btree.c:2302 at nilfs_btree_assign+0x983/0xbe0 fs/nilfs2/btree.c:2302,

Analysis found that a corrupted file has the following btree layout:
Level2(key/ptr): [ 256/15 ]
Level1(key/ptr): [ 0/8, 1/9, 0/10, 3/11, 4/12, 5/13, 6/14, 139637976727559/16, 0/17 ]

The test truncated the file to 2 bytes, which partially zeroes the first block and
adds the file to the dirty list. When the segment constructor writes it and assigns
a new blocknr for the index block, it searches the btree with key=0 and min level=2,
and apparently returns ENOENT.

Therefore, we should perform more checks on the btree nodes and return early.

Reported-by: [email protected]
Closes: https://syzkaller.appspot.com/bug?extid=158be45e4d99232e1900
Signed-off-by: Wang Jianjian <[email protected]>
---
 fs/nilfs2/btree.c | 17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)

diff --git a/fs/nilfs2/btree.c b/fs/nilfs2/btree.c
index 64d5f7c5ab44..adf087c896ba 100644
--- a/fs/nilfs2/btree.c
+++ b/fs/nilfs2/btree.c
@@ -341,7 +341,8 @@ static int nilfs_btree_node_broken(const struct nilfs_btree_node *node,
 				   sector_t blocknr)
 {
 	int level, flags, nchildren;
-	int ret = 0;
+	__u64 key, prev_key;
+	int i;
 
 	level = nilfs_btree_node_get_level(node);
 	flags = nilfs_btree_node_get_flags(node);
@@ -356,9 +357,19 @@ static int nilfs_btree_node_broken(const struct nilfs_btree_node *node,
 			   "bad btree node (ino=%llu, blocknr=%llu): level = %d, flags = 0x%x, nchildren = %d",
 			   inode->i_ino, (unsigned long long)blocknr, level,
 			   flags, nchildren);
-		ret = 1;
+		return 1;
 	}
-	return ret;
+
+	for (i = 1, prev_key = nilfs_btree_node_get_key(node, 0); i < nchildren; i++, prev_key = key) {
+		key = nilfs_btree_node_get_key(node, i);
+		if (unlikely(key <= prev_key)) {
+			nilfs_crit(inode->i_sb,
+				"bad btree node(ino=%llu, blocknr=%llu), key not sorted: %d/%llu <= %d/%llu",
+				inode->i_ino, (unsigned long long)blocknr, i, key, i - 1, prev_key);
+			return 1;
+		}
+	}
+	return 0;
 }
 
 /**
-- 
2.34.1