So much misinformation circling about this story, along with people filling in their own details as they go. Anyone interested in what actually happened should probably take a look at the
research paper in question and the relevant linux kernel maintainers'
email thread.
The researchers who wrote the paper only created three 'bad' patches. They took steps to ensure that these patches never actually made it into the kernel, as well as tried to minimize the impact on the maintainers' time. See section VI-A in the paper. So people should probably put down their pitchforks. As an aside, it wasn't two students; it was a student and a prof.
At some later date, another person/people from the university submitted some more patches. It's not clear if they have a direct relation to the two authors, other than working at the same place. These patches do not appear to be malicious, but maintainers felt that they were largely useless and/or time-wasting submissions. The submitters claim the patches were the result of the output of a new static analysis tool they were trying out, and insist they were submitted in good faith (which the maintainers didn't buy). This is, along with the 'experiment' the two researchers conducted, are what prompted Kroah-Hartman's statement.
With regard to Kroah-Hartman's statement about "rip[ping] out your previous contributions, as they were obviously submitted in bad-faith with the intent to cause problems", he was seemingly referring to all past contributions from people with umn.edu email addresses (not necessarily the author's of the paper). As far as I'm aware there's no evidence that these past submissions had (deliberate) vulnerabilities, and in the email thread one of the maintainers' even vouches for some of the past patches as being legit.