I feel that China will never allow any attack on pakistan.there are many reasons.indians talk about secret deals and no trust between China and pakistan but if you think from Chinese point of view,things will become clear.
China don't want India to expand.they and their people can never forget memories of war against India.there are other reasons as well.one of the critical question is what will happen to china if India manages to defeat pakistan? China will be doomed.from South China Sea,allies will tackle them.from indian and assumed captured pakistani territory,indians will face them.russia will never intervene in China and India war.
There are other problems like boundary problem between China and India.china don't want string of pearls near it's border and they understand importance of pakistan.trade is key for China but they can fight any war.they have money just like India.
If you look at indian Kashmir map,you can see indian Kashmir is surrounded by Pakistani and Chinese territory from at least two different sides which means two different attacks on different sectors will put indian military in panic mood and there will be no time to attack pakistani Punjab because according to Chinese military doctrine,war will be swift so I don't think that India will attack us on Punjab front.india don't have any chance in case of two front war and similar is true for pakistan as well but we know,two front on our side is almost impossible.