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

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Message-ID: <20251018053029.226506-1-hsukrut3@gmail.com>
Date: Sat, 18 Oct 2025 11:00:25 +0530
From: Sukrut Heroorkar <hsukrut3@...il.com>
To: Dave Kleikamp <shaggy@...nel.org>,
	Rand Deeb <rand.sec96@...il.com>,
	Edward Adam Davis <eadavis@...com>,
	Ghanshyam Agrawal <ghanshyam1898@...il.com>,
	Nihar Chaithanya <niharchaithanya@...il.com>,
	Vasiliy Kovalev <kovalev@...linux.org>,
	Sukrut Heroorkar <hsukrut3@...il.com>,
	Arnaud Lecomte <contact@...aud-lcm.com>,
	jfs-discussion@...ts.sourceforge.net (open list:JFS FILESYSTEM),
	linux-kernel@...r.kernel.org (open list)
Cc: skhan@...ux.foundation.org,
	david.hunter.linux@...il.com,
	syzbot+4b717071f1eecb2972df@...kaller.appspotmail.com
Subject: [PATCH] jfs: validate budmin to prevent shift-out-of-bounds in dbAllocAG()

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@...kaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=4b717071f1eecb2972df
Tested-by: syzbot+4b717071f1eecb2972df@...kaller.appspotmail.com
Signed-off-by: Sukrut Heroorkar <hsukrut3@...il.com>
---
 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