Somebody is in possession of something.

Something is in the possession of somebody.

So for example:

The passenger was in possession of a very large dog.

A very large dog was in the possession of the passenger.

Mistakes can sound very silly to native English speakers. If you write

A very large dog was in possession of the passenger

it means the dog possessed the passenger.

The easiest way to avoid such mistakes is to avoid using these phrases at all. A sentence like

We are in possession of all the required documentation

can simply be rewritten as

We have all the required documentation.

And this is what I suggest you do.