Re: [PATCH v3] ocfs2: fix kernel BUG in ocfs2_find_victim_chain

[Joseph Qi]

On 2025/12/1 21:07, 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 (next free slot in 
> the chain list) is 0, triggring the BUG_ON(!cl->cl_next_free_rec) 
> condition in ocfs2_find_victim_chain() and panicking the kernel.
> 
> To fix this, an if condition is introduced in ocfs2_claim_suballoc_bits(),
> just before calling ocfs2_find_victim_chain(), the code block in it being 
> executed when either of the following conditions is true:
> 
> 1. `cl_next_free_rec` is equal to 0, indicating that there are no free 
> chains in the allocation chain list
> 2. `cl_next_free_rec` is greater than `cl_count` (the total number of 
> chains in the allocation chain list)
> 
> Either of them being true is indicative of the fact that there are no 
> chains left for usage. 
> 
> This is addressed using ocfs2_error(), which prints
> the error log for debugging purposes, rather than panicking the kernel.
> 
> Reported-by: syzbot+96d38c6e1655c1420a72@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://syzkaller.appspot.com/bug?extid=96d38c6e1655c1420a72
> Tested-by: syzbot+96d38c6e1655c1420a72@xxxxxxxxxxxxxxxxxxxxxxxxx 
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Prithvi Tambewagh <activprithvi@xxxxxxxxx>

Reviewed-by: Joseph Qi <joseph.qi@xxxxxxxxxxxxxxxxx>
> ---
> v2->v3
>  - Revise log message for reflecting changes from v1->v2
>  - Format code style as suggested in v2
> 
> 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 | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/fs/ocfs2/suballoc.c b/fs/ocfs2/suballoc.c
> index 6ac4dcd54588..e93fc842bb20 100644
> --- a/fs/ocfs2/suballoc.c
> +++ b/fs/ocfs2/suballoc.c
> @@ -1992,6 +1992,16 @@ 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 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));
> +		goto bail;
> +	}
>  
>  	victim = ocfs2_find_victim_chain(cl);
>  	ac->ac_chain = victim;
> 
> base-commit: 939f15e640f193616691d3bcde0089760e75b0d3