Determining The Polarity of a FeedBack Circuit
You are correct, the feedback is positive. To check whether a feedback is negative or positive just imagine breaking the loop at a certain point and applying some disturbance on the node. If the returned disturbance is of the same polarity as the applied one then it is positive otherwise it is a negative feedback.
So for the circuit you showed if, for instance, we break the loop at \$V_{out}\$ and imagine the gate potential of M2 going up by a small amount. This would increase its gate source potential and \$I_{dM2}\$ increases. Since more current now flows into ground, the drain potential of M2 will go down. This would reduce the gate potential of M3 causing \$I_{dM3}\$ to go up. Since there is more current through the resistor \$V_{out}\$ goes up (same way as the applied disturbance). So we see that the feedback tries to amplify any disturbance in the circuit so it is positive feedback.
An easier way to do the same thing is to notice that while traversing the loop we move from gate of M2 to its drain, kind of like a common source, which is an inverting stage. From drain of M2 we move to drain of M3 which is again an inverting stage. Since we go through even number of inversions, the output disturbance will be in phase with the applied disturbance and feedback will be positive.