The Samsung Galaxy S8 rear-facing camera features one of these two sensors, either the S5K2L2 ISCOCELL from System SLI, a Samsung's subsidiary, or the Sony IMX333 sensor. According to a post posted by 萌萌的电教 on Twitter, Samsung approves the detail. Here is the Tweet with the camera specifications that shows SONY_IMX333 12.2MP (4032 x 3024) 4.25mm focal length for the rear camera. You can also see the specs for the front camera as well.
So the two sensors provide the same specs, which means a 12.2-megapixel resolution, dual-pixel phase-detection autofocus, 1/2.55-inch size sensor, Optical Image Stabilization and 4.25mm (non-equivalent) F1.7 lens.
So the Samsung Galaxy S8 uses a new Samsung or Sony sensor. The Galaxy S7 rear camera uses either the Samsung S5K2L1 or Sony IMX260. By the way, the specs are in accordance with the system data from the S8 phone itself. This system info definitely show a different sensors module listed there.
I first thought that this is the same exact camera sensor, but apparently it isn't. I will be interesting to see whether this new sensor outperforms the one from the Galaxy S7. I believe that the performance would be more or less the same, because if it was marginally better, Samsung would have said something about it, that's what I think.
It's great to know that the S8 comes with a new sensor compared to the Galaxy S7 and I do hope that there will be at least some improvement in the image quality, but we'll wait to wait and see.