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

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

[Prithvi Tambewagh]

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;
 
+	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

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

[Joseph Qi]

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

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

[Prithvi Tambewagh]

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?

Thanks,
Prithvi

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

[Joseph Qi]

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 

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

[Prithvi Tambewagh]

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)

Hello Joseph,

I went through the code in fs/ocfs2/suballoc.c, like this function 

static inline u16 ocfs2_find_smallest_chain(struct ocfs2_chain_list *cl)
{
	u16 curr, best;

	best = curr = 0;
	while (curr < le16_to_cpu(cl->cl_count)) {
		if (le32_to_cpu(cl->cl_recs[best].c_total) >
		    le32_to_cpu(cl->cl_recs[curr].c_total))
			best = curr;
		curr++;
	}
	return best;
}

and in function ocfs2_block_group_alloc() these lines 

if (le16_to_cpu(cl->cl_next_free_rec) < le16_to_cpu(cl->cl_count))
	le16_add_cpu(&cl->cl_next_free_rec, 1);

and observed that according to the architecture of ocfs2, the chain list is 
in the form of 0-indexed array. In that case, the change you suggested for 
upper limit, could be re-written as 

le16_to_cpu(cl->cl_next_free_rec) >= le16_to_cpu(cl->cl_count)

since value of cl->cl_next_free_rec greater than or equal to cl->cl_count 
will indicate that there are no available chains. Can you please review this?

Thank you,
Prithvi

>
>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
>

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

[Joseph Qi]

On 2025/12/1 14:24, 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)
> 
> Hello Joseph,
> 
> I went through the code in fs/ocfs2/suballoc.c, like this function
> static inline u16 ocfs2_find_smallest_chain(struct ocfs2_chain_list *cl)
> {
>     u16 curr, best;
> 
>     best = curr = 0;
>     while (curr < le16_to_cpu(cl->cl_count)) {
>         if (le32_to_cpu(cl->cl_recs[best].c_total) >
>             le32_to_cpu(cl->cl_recs[curr].c_total))
>             best = curr;
>         curr++;
>     }
>     return best;
> }
> 
> and in function ocfs2_block_group_alloc() these lines
> if (le16_to_cpu(cl->cl_next_free_rec) < le16_to_cpu(cl->cl_count))
>     le16_add_cpu(&cl->cl_next_free_rec, 1);
> 
After this, cl_next_free_rec may equal to cl_count.


> and observed that according to the architecture of ocfs2, the chain list is in the form of 0-indexed array. In that case, the change you suggested for upper limit, could be re-written as
> le16_to_cpu(cl->cl_next_free_rec) >= le16_to_cpu(cl->cl_count)
> 
> since value of cl->cl_next_free_rec greater than or equal to cl->cl_count will indicate that there are no available chains. Can you please review this?
> 
Yes, it's full. But 'cl_next_free_rec == cl_count' is a designed behavior, see mkfs or fsck.

Joseph

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

[Prithvi Tambewagh]

On Mon, Dec 01, 2025 at 03:07:56PM +0800, Joseph Qi wrote:
>
>
>On 2025/12/1 14:24, 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)
>>
>> Hello Joseph,
>>
>> I went through the code in fs/ocfs2/suballoc.c, like this function
>> static inline u16 ocfs2_find_smallest_chain(struct ocfs2_chain_list *cl)
>> {
>>     u16 curr, best;
>>
>>     best = curr = 0;
>>     while (curr < le16_to_cpu(cl->cl_count)) {
>>         if (le32_to_cpu(cl->cl_recs[best].c_total) >
>>             le32_to_cpu(cl->cl_recs[curr].c_total))
>>             best = curr;
>>         curr++;
>>     }
>>     return best;
>> }
>>
>> and in function ocfs2_block_group_alloc() these lines
>> if (le16_to_cpu(cl->cl_next_free_rec) < le16_to_cpu(cl->cl_count))
>>     le16_add_cpu(&cl->cl_next_free_rec, 1);
>>
>After this, cl_next_free_rec may equal to cl_count.
>
>
>> and observed that according to the architecture of ocfs2, the chain list is in the form of 0-indexed array. In that case, the change you suggested for upper limit, could be re-written as
>> le16_to_cpu(cl->cl_next_free_rec) >= le16_to_cpu(cl->cl_count)
>>
>> since value of cl->cl_next_free_rec greater than or equal to cl->cl_count will indicate that there are no available chains. Can you please review this?
>>
>Yes, it's full. But 'cl_next_free_rec == cl_count' is a designed behavior, see mkfs or fsck.

I get it. We are trying to catch a state of disk corruption, so your 
suggestion

le16_to_cpu(cl->cl_next_free_rec) > le16_to_cpu(cl->cl_count)

fits best here. Thanks...I will make v2 for the patch.

Best Reards,
Prithvi

>
>Joseph
>