How to change a variable used in a loop for the outside ?

Hello,

I've made a coroutine which applies a buff to a player.

It works like this :

Applying Buff :

 stoptime=Time.time+buffDuration;
 StartCoroutine("BuffAttack");

Coroutine :

 function BuffAttack();
     attack=2;
     while (Time.time<stoptime){
         yield WaitForSeconds(0.1f);
     }
     attack=1;

I want to be able to extend the duration of the buff while it is applied. I use for this the following :

Refreshing Buff :

 stoptime=Time.time+duration;

However, the stoptime in the while function is always the same. How can I deal with this ? Thanks a lot.