syzbot is already fuzzing the latest two stable kernels and has found hundreds of bugs, including lots of use-after-frees. All these bugs are listed here:
As far as I know, no one is doing anything with the syzbot bugs against stable kernels directly, since no company using Linux is paying anyone to do it as their job. But some are getting fixed; e.g., some get reported against mainline too, then fixed and backported.
Time and cost, same as it would be to do it across all kernel versions, not just current ones. Theoretically could be done pretty simply via a CI/CD pipeline if someone wrote solid test cases for the issues found by the fuzzier.
- https://syzkaller.appspot.com/linux-4.14 - https://syzkaller.appspot.com/linux-4.19
As far as I know, no one is doing anything with the syzbot bugs against stable kernels directly, since no company using Linux is paying anyone to do it as their job. But some are getting fixed; e.g., some get reported against mainline too, then fixed and backported.