They MIGHT have been smoking cannabis, but it was probably tobacco. There is no guarantee that the painting Reply of the Zaporozhian Cossacks to Sultan Mehmed IV of the Ottoman Empire is historically accurate in any sense -- it was painted in the 1880s based on a 18th century document which may or may not have been a faithful and accurate recounting of the events it depicts, which took place around 1676. That said, by the mid-17th century, tobacco was a widely-traded luxury good in Europe and the Middle East. Certainly by the end of the 17th century tobacco was cheap enough that even peasants in Europe and Asia could (and did) smoke pipes as an occasional luxury -- but not the sort of massive regular consumption that characterizing modern pack-a-day habits.