On the backside, use gel medium or lineco pH neutral glue and glue on small patch of the same canvas - gesso this patch first and just make sure the canvas has much the same treatment as the painting for shrinking purposes, etc. Just treat one side; the raw side should be glued to the backside of the torn canvas - so raw side to raw side.
On the front side, once the mediu/glue has dried, you can disguise the tear with more paint, more medium, really make it disappear. I've done this with torn paper and you can really hard to tell.