It isn't just about China or Pakistan. It is about everyone. A simple search like "provided weapon to terrorists" gives links of dozens of news articles on a particular country providing weapons to insurgents or terrorists of another country, or allegations of one country doing so.
Is it legal? I assume it isn't. At least one organisation or the other "advices" to refrain from some acts.
Once proved doing so, can anyone take actions for doing it (from the perspective of any international law)?
Just want to clarify, if the questions seem vague.
For the first, organisations that play a major role in maintaining peace may be considered.
For the second "can" or "can't" might be sounding vague. Here, I would actually want to know are there any provisions for such. If "international law" sounds vague, assume any framework of any regional grouping or general grouping of countries.
Note: I hope my questions make sense. Feel free to make a comment if there is anything more specific to be added.