The Soviet constitution, intentionally or not, was actually pretty badly written and allows a whole host of abuses to occur without violating the constitution.

An example would be:

ARTICLE 128. The inviolability of the homes of citizens and privacy of correspondence are protected by law.

Which is suppose to be roughly the equivalent of the fourth Amendment, except of course instead of simply offering protection against unreasonable search and seizure, it basically says that the government will make some set of laws to protect against it and the legislature gets to decide on what it is and what sort of protections are actually offered. And since the Communist party controls all legislation the party basically gets a free pass on deciding what kind privacy are actually protected.

So for the first decades of the USSR there were basically no laws protecting privacy, and even when laws started to be enacted in the 1950s under Khrushchev, they mostly protected private correspondences of high officials rather than ordinary citizens.

Similarly there's a provision for healthcare:

ARTICLE 120. Citizens of the U.S.S.R. have the right to maintenance in old age and also in case of sickness or loss of capacity to work. This right is ensured by the extensive development of social insurance of workers and employees at state expense, free medical service for the working people and the provision of a wide network of health resorts for the use of the working people.

The key words are of course "working people", which means that if you are someone who isn't considered part of the "working people" don't get healthcare. So it's perfectly constitutional to deny healthcare to political prisoners or gulag prisoners because they aren't considered part of the working class. So the government basically gets a free pass on deciding who actually gets healthcare and who doesn't.