[PATCH] jfs: validate budmin to prevent shift-out-of-bounds in dbAllocAG()

[Sukrut Heroorkar]

syzbot reported "UBSAN: shift-out-of-bounds in fs/jfs/jfs_dmap.c:1440:48
shift exponent -1 is negative".

The budmin value can have a negative value and cause shift-out-of-
-bounds from UBSAN.

Add a check on budmin immediately after reading it from the metapage,
and return an error if it's negative. This prevents UBSAN reports and
correctly treats corrupted metadata as an I/O error.

Reported-by: syzbot+4b717071f1eecb2972df@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=4b717071f1eecb2972df
Tested-by: syzbot+4b717071f1eecb2972df@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Sukrut Heroorkar <hsukrut3@xxxxxxxxx>
---
 fs/jfs/jfs_dmap.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/fs/jfs/jfs_dmap.c b/fs/jfs/jfs_dmap.c
index cdfa699cd7c8..76f4b9322034 100644
--- a/fs/jfs/jfs_dmap.c
+++ b/fs/jfs/jfs_dmap.c
@@ -1372,6 +1372,12 @@ dbAllocAG(struct bmap * bmp, int agno, s64 nblocks, int l2nb, s64 * results)
 	dcp = (struct dmapctl *) mp->data;
 	budmin = dcp->budmin;
 
+	if (unlikely(budmin < 0)) {
+		jfs_err("JFS: dmapctl corruption: budmin=%d", budmin);
+		release_metapage(mp);
+		return -EIO;
+	}
+
 	if (dcp->leafidx != cpu_to_le32(CTLLEAFIND)) {
 		jfs_error(bmp->db_ipbmap->i_sb, "Corrupt dmapctl page\n");
 		release_metapage(mp);
-- 
2.43.0