Instabilities of naked singularities and black hole interiors in General Relativity

Metrics representing black holes in General Relativity may exhibit naked singularities for certain values of their parameters. This is the case for super-extremal ($J^2>M>0$) Kerr and super-extremal ($\mathrm{\mid }Q\mathrm{\mid }>M>0$) Reissner-N"ordstrom spacetimes, and also for the negative mass Schwarzschild spacetime. We review our recent work where we show that these nakedly singular spacetimes are unstable under linear gravitational perturbations, a result that supports the cosmic censorship conjecture, and also that the inner stationary region beyond the inner horizon of a Kerr black hole ($J^2<M$) is linearly unstable.