A permanent lease isn't a lease, it's an annexation.

It seems likely that the continuing US occupation is in fact illegal, though maybe that's not the kind of thing that superpowers worry too much about - here is an extract from an article about the base in the San Francisco Chronicle (I found it via Wikipedia):

The 103-year-old agreement limits use of the Cuban territory to "coaling and naval purposes only," neither of which appears to cover the prison or tribunal operations. The agreement also expressly prohibits "commercial, industrial or other enterprise within said areas," but the U.S. base now sports a McDonald's, two Starbucks outlets, a Subway sandwich shop and other American concessions.

Such breaches of the treaty render it voidable, the Council on Hemispheric Affairs stated in its report urging the U.S. government to cease its use of Guantanamo against the host country's wishes.