linux-kernel - [PATCH] fs: ocfs2: fix kernel BUG in ocfs2_find_victim

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Message-Id: <20251130104637.264258-1-activprithvi@gmail.com>
Date: Sun, 30 Nov 2025 16:16:37 +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+96d38c6e1655c1420a72@...kaller.appspotmail.com,
	stable@...r.kernel.org
Subject: [PATCH] fs: ocfs2: fix kernel BUG in ocfs2_find_victim_chain

syzbot reported a kernel BUG in ocfs2_find_victim_chain() because the
`cl_next_free_rec` field of the allocation chain list is 0, triggring the
BUG_ON(!cl->cl_next_free_rec) condition and panicking the kernel.

To fix this, `cl_next_free_rec` is checked inside the caller of
ocfs2_find_victim_chain() i.e. ocfs2_claim_suballoc_bits() and if it is
equal to 0, ocfs2_error() is called, to log the corruption and force the
filesystem into read-only mode, to prevent further damage.

Reported-by: syzbot+96d38c6e1655c1420a72@...kaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=96d38c6e1655c1420a72
Tested-by: syzbot+96d38c6e1655c1420a72@...kaller.appspotmail.com
Cc: stable@...r.kernel.org
Signed-off-by: Prithvi Tambewagh <activprithvi@...il.com>
---
 fs/ocfs2/suballoc.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/fs/ocfs2/suballoc.c b/fs/ocfs2/suballoc.c
index 6ac4dcd54588..84bb2d11c2aa 100644
--- a/fs/ocfs2/suballoc.c
+++ b/fs/ocfs2/suballoc.c
@@ -1993,6 +1993,13 @@ static int ocfs2_claim_suballoc_bits(struct ocfs2_alloc_context *ac,
 
 	cl = (struct ocfs2_chain_list *) &fe->id2.i_chain;
 
+	if (le16_to_cpu(cl->cl_next_free_rec) == 0) {
+		status = ocfs2_error(ac->ac_inode->i_sb,
+				     "Chain allocator dinode %llu has 0 chains\n",
+				     (unsigned long long)le64_to_cpu(fe->i_blkno));
+		goto bail;
+	}
+
 	victim = ocfs2_find_victim_chain(cl);
 	ac->ac_chain = victim;
 

base-commit: 939f15e640f193616691d3bcde0089760e75b0d3
-- 
2.34.1