[PATCH] nilfs2: handle corrupted checkpoint count gracefully during deletion

[Igor Putko <igorpetindev@gmail.com>]

Syzkaller reported a kernel warning in nilfs_cpfile_delete_checkpoints()
due to a corrupted checkpoint count on the storage medium where
le32_to_cpu(cp->cp_checkpoints_count) is less than the number of
checkpoints being deleted.
Triggering a WARN_ON() for disk image corruption is suboptimal. Fix
this by returning -EIO and reporting a filesystem error via
nilfs_error() instead of interrupting execution with a kernel warning.

Reported-by: syzbot+79b815da3aec0a6a4d02@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=79b815da3aec0a6a4d02
Signed-off-by: Igor Putko <igorpetindev@gmail.com>
---
 fs/nilfs2/cpfile.c | 20 ++++++++++++++++----
 1 file changed, 16 insertions(+), 4 deletions(-)

diff --git a/fs/nilfs2/cpfile.c b/fs/nilfs2/cpfile.c
index 4bbdc832d..d3349fa58 100644
--- a/fs/nilfs2/cpfile.c
+++ b/fs/nilfs2/cpfile.c
@@ -81,18 +81,26 @@ nilfs_cpfile_block_add_valid_checkpoints(const struct inode *cpfile,
 	return count;
 }
 
-static unsigned int
+static int
 nilfs_cpfile_block_sub_valid_checkpoints(const struct inode *cpfile,
 					 struct buffer_head *bh,
 					 unsigned int n)
 {
 	struct nilfs_checkpoint *cp;
-	unsigned int count;
+	unsigned int checkpoints_count;
+	int count;
 
 	cp = kmap_local_folio(bh->b_folio,
 			      offset_in_folio(bh->b_folio, bh->b_data));
-	WARN_ON(le32_to_cpu(cp->cp_checkpoints_count) < n);
-	count = le32_to_cpu(cp->cp_checkpoints_count) - n;
+	checkpoints_count = le32_to_cpu(cp->cp_checkpoints_count);
+	if (unlikely(checkpoints_count < n)) {
+		nilfs_error(cpfile->i_sb,
+			    "deleted checkpoints count %u exceeds block count %u",
+			    n, checkpoints_count);
+		kunmap_local(cp);
+		return -EIO;
+	}
+	count = checkpoints_count - n;
 	cp->cp_checkpoints_count = cpu_to_le32(count);
 	kunmap_local(cp);
 	return count;
@@ -522,6 +530,10 @@ int nilfs_cpfile_delete_checkpoints(struct inode *cpfile,
 		count = nilfs_cpfile_block_sub_valid_checkpoints(cpfile, cp_bh,
 								 nicps);
 		brelse(cp_bh);
+		if (unlikely(count < 0)) {
+			ret = count;
+			break;
+		}
 		if (count)
 			continue;
 
-- 
2.47.3