clone() system call. Not earth shattering, but definitely fun. Essentially, you can send any signal you want at any time to your parent process, even if it is running with real and effective user id of someone else (e.g.
root). Full technical details and an example may be found here:
Maybe someone more devious that me can come up with better abuse scenarios than I can. Have at it...
Signals are a tricky area of the kernel on a lot of levels. I find it interesting that every slightly unusual way to send signals in the kernel has suffered from access control issues in the past. For example, this COSEINC advisory notes issues in sending signals via
prctl(PR_SET_PDEATHSIG, ...). There were multi-vendor issues with
fcntl(..., F_SETOWN, ...) a long time ago which resurfaced in a Linux-specific manner a little after.