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@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://syzkaller.appspot.com/bug?extid=96d38c6e1655c1420a72
> Tested-by: syzbot+96d38c6e1655c1420a72@xxxxxxxxxxxxxxxxxxxxxxxxx
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Prithvi Tambewagh <activprithvi@xxxxxxxxx>
> ---
>  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