That was Stalin's original plan. The Czech buffer state failed when it turned out that Stalin was the only one willing to defend it (even the Czechs weren't willing to fight without Wallied support), resulting in Munich. The Poland buffer state failed after repeated Soviet attempts to fully ally with the Wallies resulted in little outcome.
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So that's why Czech wouldn't work.
Well, the Poles aren't going to hate him any less for it and will refuse Soviet troops on their soil (although whether that would last after/if Hitler invades)...
That would be the Polish position (no Soviets) up until the Nazi invasion, and that would change pretty soon after German invasion.
The result would be a see-saw Soviet-German war in Poland with the west watching, until German collapse and getting crushed on two fronts.
In the best-case for the Nazis, worst-case for Soviets & Poles, the Germans at their apex extend their front-line as far east as the Dnepr and Dvina.
Khruschev's (or somebody else's) secret speech after Stalin's death would denounce Stalin for pulling other countries' chestnuts out of the fire at great cost to the Soviet people. Any revelation of the possibility of a partition of Poland instead will strengthen the revisionist argument in the USSR that Molotov-Ribbentropp talks were a great lost opportunity for the USSR.
In reality, the ATL USSR would be better off than the OTL USSR, but it wouldn't know that, because from their point of view the OTL course of events would only be one of many hypothetical alternative outcomes, and a pretty convoluted one at that.
The lesson the USSR takes from WWII is to stay out of alliances and wars of choice.