fs: ocfs2: fix kernel BUG in ocfs2_find_victim_chain

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.

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

index 6ac4dcd54588..c7eb6efc00b4 100644
@@ -1993,6 +1993,13 @@ 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) == 0) {
+		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