[PATCH] ocfs2: Fix kernel BUG in ocfs2_write_block

[Prithvi Tambewagh]

When the filesystem is being mounted, the kernel panics while the data
regarding slot map allocation to the local node, is being written to the
disk. This occurs because the value of slot map buffer head block
number, which should have been greater than or equal to
`OCFS2_SUPER_BLOCK_BLKNO` (evaluating to 2) is less than it, indicative
of disk metadata corruption. This triggers
BUG_ON(bh->b_blocknr < OCFS2_SUPER_BLOCK_BLKNO) in ocfs2_write_block(),
causing the kernel to panic.

This is fixed by introducing an if condition block in
ocfs2_update_disk_slot(), right before calling ocfs2_write_block(), which
checks if `bh->b_blocknr` is lesser than `OCFS2_SUPER_BLOCK_BLKNO`; if
yes, then ocfs2_error is called, which prints the error log, for
debugging purposes, and the return value of ocfs2_error() is returned
back to caller of ocfs2_update_disk_slot() i.e. ocfs2_find_slot(). If
the return value is zero. then error code EIO is returned.

Reported-by: syzbot+c818e5c4559444f88aa0@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=c818e5c4559444f88aa0
Tested-by: syzbot+c818e5c4559444f88aa0@xxxxxxxxxxxxxxxxxxxxxxxxx
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Prithvi Tambewagh <activprithvi@xxxxxxxxx>
---
 fs/ocfs2/slot_map.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/fs/ocfs2/slot_map.c b/fs/ocfs2/slot_map.c
index e544c704b583..788924fc3663 100644
--- a/fs/ocfs2/slot_map.c
+++ b/fs/ocfs2/slot_map.c
@@ -193,6 +193,16 @@ static int ocfs2_update_disk_slot(struct ocfs2_super *osb,
 	else
 		ocfs2_update_disk_slot_old(si, slot_num, &bh);
 	spin_unlock(&osb->osb_lock);
+	if (bh->b_blocknr < OCFS2_SUPER_BLOCK_BLKNO) {
+		status = ocfs2_error(osb->sb,
+				     "Invalid Slot Map Buffer Head "
+				     "Block Number : %llu, Should be >= %d",
+				     le16_to_cpu(bh->b_blocknr),
+				     le16_to_cpu((int)OCFS2_SUPER_BLOCK_BLKNO));
+		if (!status)
+			return -EIO;
+		return status;
+	}
 
 	status = ocfs2_write_block(osb, bh, INODE_CACHE(si->si_inode));
 	if (status < 0)

base-commit: 24172e0d79900908cf5ebf366600616d29c9b417
-- 
2.43.0