ocfs2: Validate chain list bits per cluster to prevent div-by-zero

The call trace shows that the div error occurs on the following line where the code sets the e_cpos member of the extent record while dividing bg_bits by the bits per cluster value from the chain list:

		rec->e_cpos = cpu_to_le32(le16_to_cpu(bg->bg_bits) /
				  le16_to_cpu(cl->cl_bpc));

Looking at the code disassembly we see the problem occurred during the divw instruction which performs a 16-bit unsigned divide operation. The main ways a divide error can occur is if:

1) the divisor is 0
2) if the quotient is too large for the designated register (overflow).

Normally the divisor being 0 is the most common cause for a division error to occur.

Focusing on the bits per cluster cl->cl_bpc (since it is the divisor) we see that cl is created in ocfs2_block_group_alloc(), cl is derived from ocfs2_dinode->id2.i_chain. To fix this issue we should verify the cl_bpc member in the chain list to ensure it is valid and non-zero.

Looking through the rest of the OCFS2 code it seems like there are other places which could benefit from improved checks of the cl_bpc members of chain lists like the following:

In ocfs2_group_extend():

	cl_bpc = le16_to_cpu(fe->id2.i_chain.cl_bpc);
	if (le16_to_cpu(group->bg_bits) / cl_bpc + new_clusters >
		le16_to_cpu(fe->id2.i_chain.cl_cpg)) {
		ret = -EINVAL;
		goto out_unlock;
	}

---
 fs/ocfs2/resize.c   | 4 ++--
 fs/ocfs2/suballoc.c | 5 +++++
 2 files changed, 7 insertions(+), 2 deletions(-)

index b0733c08ed13..22352c027ecd 100644
@@ -329,8 +329,8 @@ int ocfs2_group_extend(struct inode * inode, int new_clusters)
 	group = (struct ocfs2_group_desc *)group_bh->b_data;
 
 	cl_bpc = le16_to_cpu(fe->id2.i_chain.cl_bpc);
-	if (le16_to_cpu(group->bg_bits) / cl_bpc + new_clusters >
-		le16_to_cpu(fe->id2.i_chain.cl_cpg)) {
+	if (!cl_bpc || le16_to_cpu(group->bg_bits) / cl_bpc + new_clusters >
+		       le16_to_cpu(fe->id2.i_chain.cl_cpg)) {
 		ret = -EINVAL;
 		goto out_unlock;
 	}

index f7b483f0de2a..844cb36bd7ab 100644
@@ -671,6 +671,11 @@ static int ocfs2_block_group_alloc(struct ocfs2_super *osb,
 	BUG_ON(ocfs2_is_cluster_bitmap(alloc_inode));
 
 	cl = &fe->id2.i_chain;
+	if (!le16_to_cpu(cl->cl_bpc)) {
+		status = -EINVAL;
+		goto bail;
+	}
+
 	status = ocfs2_reserve_clusters_with_limit(osb,
 						   le16_to_cpu(cl->cl_cpg),
 						   max_block, flags, &ac);

2.39.5