dm btree: add a defensive bounds check to insert_at()

Corrupt metadata could trigger an out of bounds write.
Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/persistent-data/dm-btree.c

@@ -81,14 +81,16 @@ void inc_children(struct dm_transaction_manager *tm, struct btree_node *n,
 }
 
 static int insert_at(size_t value_size, struct btree_node *node, unsigned index,
-		      uint64_t key, void *value)
-		      __dm_written_to_disk(value)
+		     uint64_t key, void *value)
+	__dm_written_to_disk(value)
 {
 	uint32_t nr_entries = le32_to_cpu(node->header.nr_entries);
+	uint32_t max_entries = le32_to_cpu(node->header.max_entries);
 	__le64 key_le = cpu_to_le64(key);
 
 	if (index > nr_entries ||
-	    index >= le32_to_cpu(node->header.max_entries)) {
+	    index >= max_entries ||
+	    nr_entries >= max_entries) {
 		DMERR("too many entries in btree node for insert");
 		__dm_unbless_for_disk(value);
 		return -ENOMEM;