There’s a recently talked about bug in SYSRET that apparently affects a lot of operating systems, including Linux and several BSDs. It looks like DragonFly is not affected, but Matthew Dillon has put in changes just in case.
From the document you link to:
“OpenBSD and Linux were not vulnerable. Linux actually fixed the bug in 2006, with CVE-2006-0744.”
DragonFly BSD Digest