Square Taper chainsets do this, even if they get checked for tightness before sale. A 'free service' with the bike sometimes involves nothing more than tightening these fellas up.
The problem of square taper crank arms coming loose was why Shimano came along with Octalink splined bottom brackets.
I think you will be able to get away with just tightening up the cranks. A 15mm socket set is what you need, and gloves! (Or else you can smash up your hands or hurt your hand on the socket set lever.)
You can also tighten up these bolts on the ground, back brake on, with a foot on the socket set lever at the 3 o'clock setting. Do both sides and don't bother taking them off or anything like that first. Opinions vary on whether you should have any grease in there, personally I have found grease to not do any harm, but, in general no grease is the norm.
The reason your bike has the square taper design is that Shimano patented Octalink so it did not become an industry standard.
If the problem persists then you might have already 'rounded' one or other of the crank arms. With the left-hand crank you can get a spare 170mm arm, although not necessarily to match the other arm.