With the Axis presumably able to block access to the black sea through Greece, the Baltic sea through Denmark, and monitor the arctic ocean via Norway, how difficult was it to supply the Soviet Union?
There were a number of open routes.
The most famous was the Murmansk run, North of Norway. Yes, there was a bit of running the gauntlet, but the Allies seemed to do a better job of shutting the Germans in to Norway than vice versa.
Aircraft could be ferried via the Alaska Siberia route. A similar route on ships went to Vladivostok. Since the Japanese controlled that route, no weapons could be carried, but lots of food, medicine, clothing, locomotives, etc. (Remeber, Japan was neutral to the USSR, not allied with Germany)
There was also the Mid East route. Ships would stop in Iran or Iraq, and the cargo transported North to the USSR.