Can the sun shine and cast shadows upwards?
Not usually, the earth would get in the way.
But.. ..there are slight exceptions shortly after sunset. The sun has then set at ground level but for a while it continues to illuminate high clouds. If a cloud were overhead as shown (below), sun rays could slant upwards (as judged locally) to illuminate its underside. The rays come from the west and have grazed the earth's surface there.
Simple geometry show that the effect is tiny. High cirrus at 30,000ft could, in principle, be lit by rays shining upwards at an angle of a mere 3°. That calculation ignores atmospheric refraction which would reduce the angle further to ~2°.
We see clouds underlit at sunset (see the bottom below) mostly because irregularities catch the near horizontal sun rays, because skylight in the west lights them and because they are sometimes translucent.
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