ocfs2: check el->l_next_free_rec in ocfs2_get_clusters_nocache

Recently syzbot reported a use-after-free issue[1].

The root cause of the problem is that the journal inode recorded in this
file system image is corrupted.  The value of
"di->id2.i_list.l_next_free_rec" is 8193, which is greater than the value
of "di->id2.i_list.l_count" (19).

To solve this problem, an additional check should be added within
ocfs2_get_clusters_nocache().  If the check fails, an error will be
returned and the file system will be set to read-only.

[1]: https://lore.kernel.org/all/67577778.050a0220.a30f1.01bc.GAE@google.com/T/

Link: https://lkml.kernel.org/r/20250106023432.1320904-1-sunjunchao2870@gmail.com
Signed-off-by: Julian Sun <sunjunchao2870@gmail.com>
Reported-by: syzbot+2313dda4dc4885c93578@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=2313dda4dc4885c93578
Tested-by: syzbot+2313dda4dc4885c93578@syzkaller.appspotmail.com
Reviewed-by: Joseph Qi <joseph.qi@linux.alibaba.com>
Cc: Changwei Ge <gechangwei@live.cn>
Cc: Joel Becker <jlbec@evilplan.org>
Cc: Jun Piao <piaojun@huawei.com>
Cc: Junxiao Bi <junxiao.bi@oracle.com>
Cc: Mark Fasheh <mark@fasheh.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: jc2870

fs/ocfs2/extent_map.c

@@ -435,6 +435,16 @@ static int ocfs2_get_clusters_nocache(struct inode *inode,
 		}
 	}
 
+	if (le16_to_cpu(el->l_next_free_rec) > le16_to_cpu(el->l_count)) {
+		ocfs2_error(inode->i_sb,
+			    "Inode %lu has an invalid extent (next_free_rec %u, count %u)\n",
+			    inode->i_ino,
+			    le16_to_cpu(el->l_next_free_rec),
+			    le16_to_cpu(el->l_count));
+		ret = -EROFS;
+		goto out;
+	}
+
 	i = ocfs2_search_extent_list(el, v_cluster);
 	if (i == -1) {
 		/*