A Torx/E-screw socket (Size: 5) is what I used on mine...
However, what I found was my old mirrors utilized a "sheet-metal" type (self-tapping) screw on the mirror housing side and a 6 mm x 1.0 machine screw stud on the vehicle side.
My new QX56 mirrors use a 6 mm x 1.0 machine screw stud on both the mirror and vehicle sides of the stud.
One of my new mirrors came without the mounting studs, so I went to Orchard Supply Hardware and found a 6 - 1.0 X 31 mm stud that worked perfectly utilizing the factory mounting nuts (w/ built in washer).
I now have some EXTRA of both factory style mounting studs available if anyone needs them!