The simple answer is no. Governments do no guarantee against default on mortgage backed securities. Some of the mortgages that are within the pool of a mortgage backed security MIGHT be insured through CDIC or Genworth, but there's no way to tell what percentage.
A mortgage backed security is essentially this:
Lender loans out an amount in mortgage loans, let's say $100M. They then bundle these mortgages up, figure out the average interest rate (let's say 4.5%). They then sell shares in this bundle to the general populace and "suggest" a rate of return of 3.5%. Essentially they take 1% for their "work", finders fee, underwriting costs, etc. You can be guaranteed within that 'etc' there is profit.
Essentially the Lender is 'selling' the mortgages to the investors. They manage it still, but they do NOT have any risk any longer, it is entirely assumed by the buyers of the MBS.
The downside? You are locked in. MBS securities cannot be cashed out, you are buying in for a term (since the borrowers are locked into a term, so are you). Are you guaranteed 3.5%? No, if a borrower cashes out early, you aren't earning interest on their pre-payment. You are in effect, the lender in this situation, and subject to all potential losses if the mortgage does default as well. You probably only have a small share of the $100m the entire security fund is worth, so if someone with a $200k mortgage defaults, your hit is minor, maybe $100 on the total payout at the end of the MBS term.
If you go back to 2008, this is how a lot of US banks that DIDN'T underwrite mortgages went bankrupt. They bought into MBS's that were just bad bad investments, and they had no information on the properties that were being loan against.