linux-kernel - [PATCH v2] ocfs2: fix kernel BUG in ocfs2_write

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Message-ID: <20251210193257.25500-1-activprithvi@gmail.com>
Date: Thu, 11 Dec 2025 01:02:55 +0530
From: Prithvi Tambewagh <activprithvi@...il.com>
To: mark@...heh.com,
	jlbec@...lplan.org,
	joseph.qi@...ux.alibaba.com
Cc: ocfs2-devel@...ts.linux.dev,
	linux-kernel@...r.kernel.org,
	linux-kernel-mentees@...ts.linux.dev,
	skhan@...uxfoundation.org,
	david.hunter.linux@...il.com,
	khalid@...nel.org,
	Prithvi Tambewagh <activprithvi@...il.com>,
	syzbot+c818e5c4559444f88aa0@...kaller.appspotmail.com,
	stable@...r.kernel.org
Subject: [PATCH v2] ocfs2: fix kernel BUG in ocfs2_write_block

When the filesystem is being mounted, the kernel panics while the data
regarding slot map allocation to the local node, is being written to the
disk. This occurs because the value of slot map buffer head block
number, which should have been greater than or equal to
`OCFS2_SUPER_BLOCK_BLKNO` (evaluating to 2) is less than it, indicative
of disk metadata corruption. This triggers
BUG_ON(bh->b_blocknr < OCFS2_SUPER_BLOCK_BLKNO) in ocfs2_write_block(),
causing the kernel to panic.

This is fixed by introducing an if condition block in
ocfs2_update_disk_slot(), right before calling ocfs2_write_block(), which
checks if `bh->b_blocknr` is lesser than `OCFS2_SUPER_BLOCK_BLKNO`; if
yes, then ocfs2_error is called, which prints the error log, for
debugging purposes, and the return value of ocfs2_error() is returned
back to caller of ocfs2_update_disk_slot() i.e. ocfs2_find_slot(). If
the return value is zero. then error code EIO is returned.

Reported-by: syzbot+c818e5c4559444f88aa0@...kaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=c818e5c4559444f88aa0
Tested-by: syzbot+c818e5c4559444f88aa0@...kaller.appspotmail.com
Cc: stable@...r.kernel.org
Signed-off-by: Prithvi Tambewagh <activprithvi@...il.com>
---
v1->v2:
 - Remove usage of le16_to_cpu() from ocfs2_error()
 - Cast bh->b_blocknr to unsigned long long
 - Remove type casting for OCFS2_SUPER_BLOCK_BLKNO
 - Fix Sparse warnings reported in v1 by kernel test robot
 - Update title from 'ocfs2: Fix kernel BUG in ocfs2_write_block' to
   'ocfs2: fix kernel BUG in ocfs2_write_block'

v1 link: https://lore.kernel.org/all/20251206154819.175479-1-activprithvi@gmail.com/T/

 fs/ocfs2/slot_map.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/fs/ocfs2/slot_map.c b/fs/ocfs2/slot_map.c
index e544c704b583..e916a2e8f92d 100644
--- a/fs/ocfs2/slot_map.c
+++ b/fs/ocfs2/slot_map.c
@@ -193,6 +193,16 @@ static int ocfs2_update_disk_slot(struct ocfs2_super *osb,
 	else
 		ocfs2_update_disk_slot_old(si, slot_num, &bh);
 	spin_unlock(&osb->osb_lock);
+	if (bh->b_blocknr < OCFS2_SUPER_BLOCK_BLKNO) {
+		status = ocfs2_error(osb->sb,
+				     "Invalid Slot Map Buffer Head "
+				     "Block Number : %llu, Should be >= %d",
+				     (unsigned long long)bh->b_blocknr,
+				     OCFS2_SUPER_BLOCK_BLKNO);
+		if (!status)
+			return -EIO;
+		return status;
+	}

 	status = ocfs2_write_block(osb, bh, INODE_CACHE(si->si_inode));
 	if (status < 0)

base-commit: 24172e0d79900908cf5ebf366600616d29c9b417
-- 
2.43.0