Fixing a hole in a canvas is tricky. You could use duct tape on the back if yo are not too concerned. But the professional way is a lot more complicated.
Molding Paste and a piece of canvas.
1) Cut any loose threads at the tear.
2) Cut a separate piece of scrap canvas an inch or so larger than the tear.
3) With the painting face down, apply molding paste to the piece of scrap canvas and press it onto the tear.
4) When it sticks, turn canvas over face up and remove any molding paste that has squeezed through the tear.
5) Let dry for 24 hours
6) Now the patch is in place on the back. However, on the front painted side, the tear will leave a little cavity where the tear is. So you must artfully and gently fill in the cavity with a tiny amount of molding paste so the molding paste dries level with the surrounding canvas.
7) After the molding paste in the tear hole on the front of the canvas is dry, repaint that area to match the surrounding area.