Mecanim : use mask only in certain conditions

Hello ! I am having trouble finding the right words to explain my problem, because I'm sure some people already solved this basic problem.

I am just starting to use mecanim (I know, I'm a bit late) and am fiddling around with it.

I have three animations : Idle, Walk and Punch. What I want to do is simple. My character is able to Punch while Walking (in which case, I want the punch animation to only play on his armes and torso), but also to Punch while being Idle (in which case, I want the punch animation to play on his whole body).

Currently, my setup is like so :

If the Speed parameter is >= 1, Run starts playing.

If the Attack parameter is true, Attack plays.

I thought about having the Attack animation linked to the Idle animation, in the "Whole Body" layer, but it doesn't work ; both Attack animations aren't necessarily synchronized.

How do I make my Attack animation play on the whole body if the character isn't running ?

Thanks !