What CMPicon grade was it? if it was rack or field grade they won't be helping, but will sell you a replacement stock. If it was "Service Grade", and the crack doesn't go all the way through, they may have let it pass. Hard to tell without a picture how bad it is. If they just didn't notice it and it is unserviceable, I have to think they will take care of you, since the whole point of the CMP service grade is good enough to have been issued to the troops and used dependably.
For reference, my service grade came with an all new stock and handgaurds - not new-old-stock, but brand new very well fitted replacement stock. I expect the 2 service grades I ordered back in Oct. will have new wood too. I imagine that original stocks would get swapped over to rifles of higher quality that would be more valuable with a good original stock that is still serviceable.