The passive form could be: What was known by me?
The Passive Voice is a form of a sentence in which the actual doer of an action is either not mentioned or added to the sentence using by clause, and the object of the action becomes the subject of Passive Voice sentence.
Active Voice: My father built this house in 1985.
Passive Voice: This house was built in 1985 (by my father).
In the Active Voice sentence the subject is the person who built the house.
In the Passive Voice sentence the object of the action (house) became the subject, the doer is either absent or can be introduced using by (as shown in brackets).