It's a mega-kernel, so you'll get poor occupancy past the first bounce. A better strategy is to shoot, sort, and repeat, which then also allows you to squeeze in an adaptive sampler in the middle.

> // No idea where negative values come from :(

I don't know, but:

> newRay.origin += sign(dot(newRay.direction, geometryNormal)) * geometryNormal * 1e-4;

The new origin should be along the reflected ray, not along the direction of the normal. This line basically adds the normal (with a sign) to the origin (intersection point), which seems odd.

Poor's man way to find where the negatives come from is to max(0,...) stuff until you find it.