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Subject: Re: [syzbot] UBSAN: shift-out-of-bounds in jfs_statfs
Author: tristmd@gmail.com

#syz test: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
>From 88957226783011487a1627b69a6295cb9aa7e5ac Mon Sep 17 00:00:00 2001
From: Tristan Madani <tristan@talencesecurity.com>
Date: Fri, 17 Apr 2026 16:15:13 +0000
Subject: [PATCH] jfs: validate l2nbperiext in diMount() to prevent
 shift-out-of-bounds
A corrupted filesystem image can set im_l2nbperiext to an arbitrary
value. When this value exceeds the width of an integer type, a shift
operation using it triggers UBSAN shift-out-of-bounds.
Add validation in diMount() to reject im_l2nbperiext values that are
negative, exceed 30, or are inconsistent with im_nbperiext.
Reported-by: syzbot+13ba7f3e9a17f77250fe@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=13ba7f3e9a17f77250fe
Signed-off-by: Tristan Madani <tristan@talencesecurity.com>
---
 fs/jfs/jfs_imap.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)
diff --git a/fs/jfs/jfs_imap.c b/fs/jfs/jfs_imap.c
index b84ba4d..eafbd2b 100644
--- a/fs/jfs/jfs_imap.c
+++ b/fs/jfs/jfs_imap.c
@@ -124,6 +124,18 @@ int diMount(struct inode *ipimap)
 	atomic_set(&imap->im_numfree, le32_to_cpu(dinom_le->in_numfree));
 	imap->im_nbperiext = le32_to_cpu(dinom_le->in_nbperiext);
 	imap->im_l2nbperiext = le32_to_cpu(dinom_le->in_l2nbperiext);
+
+	if (imap->im_l2nbperiext < 0 ||
+	    imap->im_l2nbperiext > 30 ||
+	    imap->im_nbperiext != (1 << imap->im_l2nbperiext)) {
+		jfs_err("diMount: invalid imap parameters: "
+			"nbperiext(%d) l2nbperiext(%d)",
+			imap->im_nbperiext, imap->im_l2nbperiext);
+		release_metapage(mp);
+		kfree(imap);
+		return -EINVAL;
+	}
+
 	for (index = 0; index < MAXAG; index++) {
 		imap->im_agctl[index].inofree =
 		    le32_to_cpu(dinom_le->in_agctl[index].inofree);
-- 
2.47.3