Re: [PATCH] fs: ocfs2: fix kernel BUG in ocfs2_find_victim_chain

[Joseph Qi <joseph.qi@linux.alibaba.com>]

On 2025/12/1 13:50, Prithvi Tambewagh wrote:
> On Mon, Dec 01, 2025 at 10:51:49AM +0800, Joseph Qi wrote:
>>
>>
>> On 2025/11/30 18:46, Prithvi Tambewagh wrote:
>>> 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@syzkaller.appspotmail.com
>>> Closes: https://syzkaller.appspot.com/bug?extid=96d38c6e1655c1420a72
>>> Tested-by: syzbot+96d38c6e1655c1420a72@syzkaller.appspotmail.com
>>> Cc: stable@vger.kernel.org
>>> Signed-off-by: Prithvi Tambewagh <activprithvi@gmail.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;
>>>
>>
>> This blank line can be eliminated.
>>
>>> +    if (le16_to_cpu(cl->cl_next_free_rec) == 0) {
>>
>> Better to add the upper limit check as well. e.g.
>>
>> !le16_to_cpu(cl->cl_next_free_rec) ||
>> le16_to_cpu(cl->cl_next_free_rec) > le16_to_cpu(cl->cl_count)
>>
>> Thanks,
>> Joseph
>>
>>> +        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
>>
> 
> Hello Joseph,
> 
> Thanks for the feedback! I will make the changes in the patch. I wanted to confirm that the new patch should be sent as v2 on this same thread, along with the change log since v1, right?
> 

No need on the same thread, just v2 with the change log since v1 is enough.

Joseph