China has trouble keeping its own people under control; even if they decide to give up on the DPRK, which Trump’s pressure seems to be showing progress on, it’s questionable to what degree they’ll be able to cut them off. For example, the Chinese have uncovered smuggling tunnels on their border with the DPRK that locals use to trade with DPRK-produced narcotics and for weapons smuggling (the latter of particular concern to China due to Uyghur militants). The DPRK will sell anything to anyone if it will give them money.
Historically their annoyances with the DPRK’s behavior have been less than their fear of having US troops on their border. But that situation may be changing.
That said, Russia is a different story. They seem to be doubling down on their support of the DPRK; they seem to love them being a thorn in the US’s side. I wouldn’t be surprised if they sent them some modern air defense systems (North Korea’s air defense network is dense but horribly obsolete).
Let's hope it isn't the "black market" for nukes.