fs/jfs: Add check for negative db_l2nbperpage

[ Upstream commit 525b861 ]

l2nbperpage is log2(number of blks per page), and the minimum legal
value should be 0, not negative.

In the case of l2nbperpage being negative, an error will occur
when subsequently used as shift exponent.

Syzbot reported this bug:

UBSAN: shift-out-of-bounds in fs/jfs/jfs_dmap.c:799:12
shift exponent -16777216 is negative

Reported-by: syzbot+debee9ab7ae2b34b0307@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=debee9ab7ae2b34b0307
Signed-off-by: Juntong Deng <juntong.deng@outlook.com>
Signed-off-by: Dave Kleikamp <dave.kleikamp@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>

fs/jfs/jfs_dmap.c

@@ -180,7 +180,8 @@ int dbMount(struct inode *ipbmap)
 	bmp->db_nfree = le64_to_cpu(dbmp_le->dn_nfree);
 
 	bmp->db_l2nbperpage = le32_to_cpu(dbmp_le->dn_l2nbperpage);
-	if (bmp->db_l2nbperpage > L2PSIZE - L2MINBLOCKSIZE) {
+	if (bmp->db_l2nbperpage > L2PSIZE - L2MINBLOCKSIZE ||
+		bmp->db_l2nbperpage < 0) {
 		err = -EINVAL;
 		goto err_release_metapage;
 	}