From what I under stand, the minimum requirement is a 6months or more validity in passport. One of the reason being, when they stamp the permanent residency visa, they show the expiry of the visa as per the expiry on your passport. For example if your passport is expiring on may 2018, your pr visa will have an expiry of may 2018 as well. I am not an expert on this, but I believe this is the case. best case scenario , is to email CIC and check with them as to the best solution.
In my case, I realized my passport is expiring and I renewed it before I applied for express entry. And while doing the passport transmission they took my new passport and asked my expired passport for reference considering the new one is fairly new. Hope this helps