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>
---
v1->v2:
 - Remove extra line before the if statement in patch
 - Add upper limit check for cl->cl_next_free_rec in the if condition

 fs/ocfs2/suballoc.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/fs/ocfs2/suballoc.c b/fs/ocfs2/suballoc.c
index 6ac4dcd54588..1257c39c2c11 100644
--- a/fs/ocfs2/suballoc.c
+++ b/fs/ocfs2/suballoc.c
@@ -1992,6 +1992,13 @@ 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) ||
+	    le16_to_cpu(cl->cl_next_free_rec) > le16_to_cpu(cl->cl_count)) {
+		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

On 2025/12/1 17:24, 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>
> ---
> v1->v2:
>  - Remove extra line before the if statement in patch
>  - Add upper limit check for cl->cl_next_free_rec in the if condition
> 
>  fs/ocfs2/suballoc.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/fs/ocfs2/suballoc.c b/fs/ocfs2/suballoc.c
> index 6ac4dcd54588..1257c39c2c11 100644
> --- a/fs/ocfs2/suballoc.c
> +++ b/fs/ocfs2/suballoc.c
> @@ -1992,6 +1992,13 @@ 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) ||
> +	    le16_to_cpu(cl->cl_next_free_rec) > le16_to_cpu(cl->cl_count)) {
> +		status = ocfs2_error(ac->ac_inode->i_sb,
> +					"Chain allocator dinode %llu has 0 chains\n",

The log message also has to be updateed. How about:

if (!le16_to_cpu(cl->cl_next_free_rec) ||
    le16_to_cpu(cl->cl_next_free_rec) > le16_to_cpu(cl->cl_count)) {
	status = ocfs2_error(ac->ac_inode->i_sb,
			     "Chain allocator dinode %llu has invalid next "
			     "free chain record %u, but only %u total\n",
			     (unsigned long long)le64_to_cpu(fe->i_blkno),
			     le16_to_cpu(cl->cl_next_free_rec),
			     le16_to_cpu(cl->cl_count));

Joseph

> +					(unsigned long long)le64_to_cpu(fe->i_blkno));
> +		goto bail;
> +	}
>  
>  	victim = ocfs2_find_victim_chain(cl);
>  	ac->ac_chain = victim;
> 
> base-commit: 939f15e640f193616691d3bcde0089760e75b0d3

On Mon, Dec 01, 2025 at 05:56:43PM +0800, Joseph Qi wrote:
>
>
>On 2025/12/1 17:24, 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>
>> ---
>> v1->v2:
>>  - Remove extra line before the if statement in patch
>>  - Add upper limit check for cl->cl_next_free_rec in the if condition
>>
>>  fs/ocfs2/suballoc.c | 7 +++++++
>>  1 file changed, 7 insertions(+)
>>
>> diff --git a/fs/ocfs2/suballoc.c b/fs/ocfs2/suballoc.c
>> index 6ac4dcd54588..1257c39c2c11 100644
>> --- a/fs/ocfs2/suballoc.c
>> +++ b/fs/ocfs2/suballoc.c
>> @@ -1992,6 +1992,13 @@ 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) ||
>> +	    le16_to_cpu(cl->cl_next_free_rec) > le16_to_cpu(cl->cl_count)) {
>> +		status = ocfs2_error(ac->ac_inode->i_sb,
>> +					"Chain allocator dinode %llu has 0 chains\n",
>
>The log message also has to be updateed. How about:

Apologies for the mistake. I will rectify it.  

>
>if (!le16_to_cpu(cl->cl_next_free_rec) ||
>    le16_to_cpu(cl->cl_next_free_rec) > le16_to_cpu(cl->cl_count)) {
>	status = ocfs2_error(ac->ac_inode->i_sb,
>			     "Chain allocator dinode %llu has invalid next "
>			     "free chain record %u, but only %u total\n",
>			     (unsigned long long)le64_to_cpu(fe->i_blkno),
>			     le16_to_cpu(cl->cl_next_free_rec),
>			     le16_to_cpu(cl->cl_count));

Thanks for the feedback! I will make v3 for the patch with these suggested 
changes.

>
>Joseph
>
>> +					(unsigned long long)le64_to_cpu(fe->i_blkno));
>> +		goto bail;
>> +	}
>>
>>  	victim = ocfs2_find_victim_chain(cl);
>>  	ac->ac_chain = victim;
>>
>> base-commit: 939f15e640f193616691d3bcde0089760e75b0d3
>

Best Regards,
Prithvi