Drain soaked mushrooms and rinse under water, chop. Strain soaking liquid to remove any grit and reserve for later.
Bring salted water to a boil for the pasta. Add the pasta to boiling water and cook for 10 minutes or until al dente.
Meanwhile, heat the olive oil in a saucepan. When hot, add garlic and saute a few seconds until aromatic, add chopped porcini mushrooms, reserved soaking liquid and tomatoes and simmer over low heat for 5 to 10 minutes.
Add cannellini beans, simmer for 5 to 10 minutes until heated through and have absorbed some of the flavor of the other ingredients. Season to taste with salt and pepper; if sauce is too thick, add some water to thin; if too thin, simmer down until properly thickened. Drain pasta, transfer to serving bowl and spoon sauce over pasta.