syzbot has found a kernel BUG in ocfs2_write_cluster_by_desc [1].

The issue arises because ocfs2_insert_extent receives start_blk
as 0, which incorrectly maps to a physical address of 0. This
occurs when block is 0 after the call to ocfs2_clusters_to_blocks
which is invoked inside the ocfs2_add_clusters_in_btree. The block
value is then passed to ocfs2_insert_extent, leading to the problem.

[1] https://syzkaller.appspot.com/bug?extid=18a87160c7d64ba2e2f6

Reported-and-tested-by: syzbot+18a87160c7d64ba2e2f6@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=18a87160c7d64ba2e2f6
Signed-off-by: Mohammed Anees <pvmohammedanees2003@gmail.com>
---
 fs/ocfs2/alloc.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/fs/ocfs2/alloc.c b/fs/ocfs2/alloc.c
index 395e23920..926ffeed8 100644
--- a/fs/ocfs2/alloc.c
+++ b/fs/ocfs2/alloc.c
@@ -4843,6 +4843,13 @@ int ocfs2_add_clusters_in_btree(handle_t *handle,
 	}
 
 	block = ocfs2_clusters_to_blocks(osb->sb, bit_off);
+	if (block == 0) {
+		mlog(ML_ERROR, "Conversion resulted in zero block number");
+		status = -EIO;
+		need_free = 1;
+		goto bail;
+	}
+
 	trace_ocfs2_add_clusters_in_btree(
 	     (unsigned long long)ocfs2_metadata_cache_owner(et->et_ci),
 	     bit_off, num_bits);
-- 
2.46.0

On 9/19/24 01:20, Mohammed Anees wrote:
> syzbot has found a kernel BUG in ocfs2_write_cluster_by_desc [1].
> 
> The issue arises because ocfs2_insert_extent receives start_blk
> as 0, which incorrectly maps to a physical address of 0. This
> occurs when block is 0 after the call to ocfs2_clusters_to_blocks
> which is invoked inside the ocfs2_add_clusters_in_btree. The block
> value is then passed to ocfs2_insert_extent, leading to the problem.
> 
> [1] https://syzkaller.appspot.com/bug?extid=18a87160c7d64ba2e2f6
> 
> Reported-and-tested-by: syzbot+18a87160c7d64ba2e2f6@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=18a87160c7d64ba2e2f6
> Signed-off-by: Mohammed Anees <pvmohammedanees2003@gmail.com>
> ---
>   fs/ocfs2/alloc.c | 7 +++++++
>   1 file changed, 7 insertions(+)
> 
> diff --git a/fs/ocfs2/alloc.c b/fs/ocfs2/alloc.c
> index 395e23920..926ffeed8 100644
> --- a/fs/ocfs2/alloc.c
> +++ b/fs/ocfs2/alloc.c
> @@ -4843,6 +4843,13 @@ int ocfs2_add_clusters_in_btree(handle_t *handle,
>   	}
>   
>   	block = ocfs2_clusters_to_blocks(osb->sb, bit_off);> +	if (block == 0) {
> +		mlog(ML_ERROR, "Conversion resulted in zero block number");
> +		status = -EIO;
> +		need_free = 1;
> +		goto bail;
> +	}
> +

If you check this function, there is no IO operation, so -EIO is not suitable.
In the the ocfs2_clusters_to_blocks() code, there are two possible cases where
the result is zero:
bit_off is 0 or bit_off is out of range for a u64 after a bit shift.
It seems that the root cause is that __ocfs2_claim_clusters allocates an
incorrect bit_off.

-Heming

>   	trace_ocfs2_add_clusters_in_btree(
>   	     (unsigned long long)ocfs2_metadata_cache_owner(et->et_ci),
>   	     bit_off, num_bits);

Yes, you are absolutely right, __ocfs2_claim_clusters indeed
allocates 0 as the bit_off, looking into this I believe the
problem is triggered due to ocfs2_search_chain called by
ocfs2_claim_suballoc_bits, what do you think would be the best
approach to solve this issue, what function I be looking for,
any insights will be highly appreciated.

Thanks!