[PATCH] fs: jfs: fix shift-out-of-bounds in dbDiscardAG

[Hoi Pok Wu]

Bug reported by syzbot, UBSAN: array-index-out-of-bounds in dbDiscardAG.
Fixed by simply rearranging agno_end smaller than MAXAG. However, I wonder if it
is a better idea to return -EINVAL, rejecting user input. Thank you.

Reported-by: syzbot+f0e0fcf3cd1047ae60ad@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Hoi Pok Wu <wuhoipok@xxxxxxxxx>
---
 fs/jfs/jfs_discard.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/fs/jfs/jfs_discard.c b/fs/jfs/jfs_discard.c
index 575cb2ba74fc..3101eaf3098a 100644
--- a/fs/jfs/jfs_discard.c
+++ b/fs/jfs/jfs_discard.c
@@ -96,6 +96,10 @@ int jfs_ioc_trim(struct inode *ip, struct fstrim_range *range)
 	 */
 	agno = BLKTOAG(start, JFS_SBI(ip->i_sb));
 	agno_end = BLKTOAG(end, JFS_SBI(ip->i_sb));
+
+	if (agno_end >= MAXAG)
+		agno_end = MAXAG - 1;
+
 	while (agno <= agno_end) {
 		trimmed += dbDiscardAG(ip, agno, minlen);
 		agno++;
-- 
2.38.0