This question is an open theological question; it is not a matter of doctrine. The Magisterium has not decided the question as to whether or not canonizations are infallible. It is the opinion of the majority of theologians that canonizations done by the Holy Father enter within the limits of his infallible teaching authority. In other words, the answer to this question is a matter of theological opinion, not a doctrine or dogma. There are no magisterial documents teaching that canonizations are infallible. The ordinary non-infallible Magisterium has no such teaching. The Popes and Councils of the Church have no such teaching. Regardless of how many or which theologians opined one way or the other on this topic, it is an open question.
See my article: Papal Infallibility and the Canonization of Saints
All of the teachings of the Magisterium are truths found explicitly or implicitly in Sacred Tradition or Sacred Scripture. The Magisterium can teach from natural law, but all of the truths of natural law are also found, at least implicitly, in Tradition and Scripture. So all that the Magisterium teaches must be found in Tradition or Scripture. Now according to the Second Vatican Council, Papal Infallibility “extends as far as the deposit of Revelation extends” (Lumen Gentium, n. 25). So unless a Saint is mentioned in Divine Revelation, his or her canonization cannot be an infallible teaching under Papal Infallibility.
My theological opinion is that canonizations (other than the Saints in Sacred Scripture) cannot fall under Papal Infallibility because the assertion that a particular person is a Saint is a judgment of the prudential order, not a teaching from Divine Revelation. The process for canonization does not examine Tradition and Scripture, to see if the individual is a Saint. Rather, the process judges temporal facts and personal testimony in order to reach a conclusion about the holiness of an individual. Such a decision is inherently prudential in nature, and therefore not a dogma or doctrine of the Church.