ocfs2: check tl_used after reading it from trancate log inode

The fuzz image has a truncate log inode whose tl_used is bigger than
tl_count so it triggers the BUG in ocfs2_truncate_log_needs_flush() [1].

As what the check in ocfs2_truncate_log_needs_flush() does, just do same
check into ocfs2_get_truncate_log_info() when truncate log inode is
reading in so we can bail out earlier.

[1]
(syz.0.17,5491,0):ocfs2_truncate_log_needs_flush:5830 ERROR: bug expression: le16_to_cpu(tl->tl_used) > le16_to_cpu(tl->tl_count)
kernel BUG at fs/ocfs2/alloc.c:5830!
RIP: 0010:ocfs2_truncate_log_needs_flush fs/ocfs2/alloc.c:5827 [inline]
Call Trace:
 ocfs2_commit_truncate+0xb64/0x21d0 fs/ocfs2/alloc.c:7372
 ocfs2_truncate_file+0xca2/0x1420 fs/ocfs2/file.c:509
 ocfs2_setattr+0x1520/0x1b40 fs/ocfs2/file.c:1212
 notify_change+0xc1a/0xf40 fs/attr.c:546
 do_truncate+0x1a4/0x220 fs/open.c:68

Link: https://lkml.kernel.org/r/tencent_B24B1C1BE225DCBA44BB6933AB9E1B1B0708@qq.com
Reported-by: syzbot+f82afc4d4e74d0ef7a89@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=f82afc4d4e74d0ef7a89
Tested-by: syzbot+f82afc4d4e74d0ef7a89@syzkaller.appspotmail.com
Signed-off-by: Edward Adam Davis <eadavis@qq.com>
Reviewed-by: Joseph Qi <joseph.qi@linux.alibaba.com>
Cc: Mark Fasheh <mark@fasheh.com>
Cc: Joel Becker <jlbec@evilplan.org>
Cc: Junxiao Bi <junxiao.bi@oracle.com>
Cc: Changwei Ge <gechangwei@live.cn>
Cc: Jun Piao <piaojun@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: ea1davis

fs/ocfs2/alloc.c

@@ -6164,7 +6164,7 @@ static int ocfs2_get_truncate_log_info(struct ocfs2_super *osb,
 	struct buffer_head *bh = NULL;
 	struct ocfs2_dinode *di;
 	struct ocfs2_truncate_log *tl;
-	unsigned int tl_count;
+	unsigned int tl_count, tl_used;
 
 	inode = ocfs2_get_system_file_inode(osb,
 					   TRUNCATE_LOG_SYSTEM_INODE,
@@ -6185,8 +6185,10 @@ static int ocfs2_get_truncate_log_info(struct ocfs2_super *osb,
 	di = (struct ocfs2_dinode *)bh->b_data;
 	tl = &di->id2.i_dealloc;
 	tl_count = le16_to_cpu(tl->tl_count);
+	tl_used = le16_to_cpu(tl->tl_used);
 	if (unlikely(tl_count > ocfs2_truncate_recs_per_inode(osb->sb) ||
-		     tl_count == 0)) {
+		     tl_count == 0 ||
+		     tl_used > tl_count)) {
 		status = -EFSCORRUPTED;
 		iput(inode);
 		brelse(bh);